add firmware inventory support

Add support to display the firmware version using the Firmware version
register from EM configuration. The version will be exposed as
xyz.openbmc_project.Software.Version interface on the Dbus.

Tested:
Unit Test -
```
> meson test -t 10 -C builddir/ --print-errorlogs --wrapper="valgrind --error-exitcode=1" test_firmware
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/1 test_firmware        OK               3.37s

Ok:                1
Fail:              0
```

Tested on Qemu -
```
> busctl tree xyz.openbmc_project.ModbusRTU
`- /xyz
  `- /xyz/openbmc_project
    |- /xyz/openbmc_project/inventory_source
    | |- /xyz/openbmc_project/inventory_source/Heat_Exchanger_12_DevTTYUSB0
    | |- /xyz/openbmc_project/inventory_source/Heat_Exchanger_12_DevTTYUSB1
    | |- /xyz/openbmc_project/inventory_source/Reservoir_Pumping_Unit_12_DevTTYUSB0
    | `- /xyz/openbmc_project/inventory_source/Reservoir_Pumping_Unit_12_DevTTYUSB1
    |- /xyz/openbmc_project/sensors
    | `- /xyz/openbmc_project/sensors/temperature
    |   |- /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_Coolant_Inlet_Temp_C
    |   |- /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_Coolant_Outlet_Temp_C
    |   |- /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_Coolant_Inlet_Temp_C
    |   `- /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_Coolant_Outlet_Temp_C
    `- /xyz/openbmc_project/software
      |- /xyz/openbmc_project/software/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_PLC_FW_Revision_9071
      `- /xyz/openbmc_project/software/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_PLC_FW_Revision_8053

> busctl  introspect xyz.openbmc_project.ModbusRTU /xyz/openbmc_project/software/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_PLC_FW_Revision_9071
NAME                                        TYPE      SIGNATURE RESULT/VALUE                             FLAGS
org.freedesktop.DBus.Introspectable         interface -         -                                        -
.Introspect                                 method    -         s                                        -
org.freedesktop.DBus.Peer                   interface -         -                                        -
.GetMachineId                               method    -         s                                        -
.Ping                                       method    -         -                                        -
org.freedesktop.DBus.Properties             interface -         -                                        -
.Get                                        method    ss        v                                        -
.GetAll                                     method    s         a{sv}                                    -
.Set                                        method    ssv       -                                        -
.PropertiesChanged                          signal    sa{sv}as  -                                        -
xyz.openbmc_project.Association.Definitions interface -         -                                        -
.Associations                               property  a(sss)    1 "running" "ran_on" "/xyz/openbmc_pr... emits-change writable
xyz.openbmc_project.Software.Activation     interface -         -                                        -
.Activation                                 property  s         "xyz.openbmc_project.Software.Activat... emits-change writable
.RequestedActivation                        property  s         "xyz.openbmc_project.Software.Activat... emits-change writable
xyz.openbmc_project.Software.Version        interface -         -                                        -
.Purpose                                    property  s         "xyz.openbmc_project.Software.Version... emits-change writable
.Version                                    property  s         "ABABABAB"                               emits-change writable
```

Change-Id: I985e12ef88547585cca93569b083f347e74a8695
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/rtu/firmware/device_firmware.hpp b/rtu/firmware/device_firmware.hpp
new file mode 100644
index 0000000..c3839f4
--- /dev/null
+++ b/rtu/firmware/device_firmware.hpp
@@ -0,0 +1,55 @@
+#pragma once
+
+#include "device/base_config.hpp"
+#include "port/base_port.hpp"
+
+#include <sdbusplus/async.hpp>
+#include <sdbusplus/async/server.hpp>
+#include <xyz/openbmc_project/Association/Definitions/aserver.hpp>
+#include <xyz/openbmc_project/Software/Activation/aserver.hpp>
+#include <xyz/openbmc_project/Software/Version/aserver.hpp>
+
+namespace phosphor::modbus::rtu::device
+{
+
+namespace config
+{
+
+struct Config;
+
+} // namespace config
+
+namespace config_intf = phosphor::modbus::rtu::device::config;
+using PortIntf = phosphor::modbus::rtu::port::BasePort;
+
+class DeviceFirmware;
+
+using FirmwareIntf = sdbusplus::async::server_t<
+    DeviceFirmware, sdbusplus::aserver::xyz::openbmc_project::software::Version,
+    sdbusplus::aserver::xyz::openbmc_project::software::Activation,
+    sdbusplus::aserver::xyz::openbmc_project::association::Definitions>;
+
+class DeviceFirmware
+{
+  public:
+    DeviceFirmware() = delete;
+
+    explicit DeviceFirmware(sdbusplus::async::context& ctx,
+                            const config_intf::Config& config,
+                            PortIntf& serialPort);
+
+    auto readVersionRegister() -> sdbusplus::async::task<void>;
+
+  protected:
+    // Object path of current firmware object
+    // TODO: check if its possible to get rid off this via mocking since its
+    // only used in tests
+    const sdbusplus::message::object_path objectPath;
+
+  private:
+    std::unique_ptr<FirmwareIntf> currentFirmware;
+    const config_intf::Config config;
+    PortIntf& serialPort;
+};
+
+} // namespace phosphor::modbus::rtu::device