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/tests/modbus_server_tester.hpp b/tests/modbus_server_tester.hpp
index ccfa204..cd3f62b 100644
--- a/tests/modbus_server_tester.hpp
+++ b/tests/modbus_server_tester.hpp
@@ -39,6 +39,13 @@
 const std::vector<uint16_t> testReadHoldingRegisterTempSigned = {
     0xFFB0}; // -80.0
 
+// Device Firmware Testing Constants
+constexpr uint16_t testReadHoldingRegisterFirmwareVersionOffset = 0x0115;
+constexpr uint16_t testReadHoldingRegisterFirmwareVersionCount = 0x2;
+const std::vector<uint16_t> testReadHoldingRegisterFirmwareVersion = {
+    0x5244, 0x4630};
+constexpr std::string testReadHoldingRegisterFirmwareVersionStr = "RDF0";
+
 static const std::map<uint16_t, std::tuple<uint16_t, std::vector<uint16_t>>>
     testReadHoldingRegisterMap = {
         {testSuccessReadHoldingRegisterOffset,
@@ -54,6 +61,9 @@
           testReadHoldingRegisterTempUnsigned}},
         {testReadHoldingRegisterTempSignedOffset,
          {testReadHoldingRegisterTempCount, testReadHoldingRegisterTempSigned}},
+        {testReadHoldingRegisterFirmwareVersionOffset,
+         {testReadHoldingRegisterFirmwareVersionCount,
+          testReadHoldingRegisterFirmwareVersion}},
 };
 
 class ServerTester