chassishanlder: restorePolicy supports Unknown type

The PDI interface of RestorePolicy already supports None type [1],
and ipmitool will resolve it to 0x03 by default. This commit is
adding None type.

[1] https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/yaml/xyz/openbmc_project/Control/Power/RestorePolicy.interface.yaml#L24

Tested: If the value of the PowerRestorePolicy attribute on DBus is
equal to xyz.openbmc_project.Control.Power.RestorePolicy.Policy.None

1. without this patch:
~# ipmitool chassis status
Error sending Chassis Status command: Unspecified error

2. with this patch:
~# ipmitool chassis status
System Power         : off
Power Overload       : false
Power Interlock      : inactive
Main Power Fault     : false
Power Control Fault  : false
Power Restore Policy : unknown
Last Power Event     :
Chassis Intrusion    : inactive
Front-Panel Lockout  : inactive
Drive Fault          : false
Cooling/Fan Fault    : false
Front Panel Control  : none

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I91ae1c40229c8646846443c0977685241f456838
diff --git a/chassishandler.cpp b/chassishandler.cpp
index 414d6cb..c7c1fed 100644
--- a/chassishandler.cpp
+++ b/chassishandler.cpp
@@ -930,7 +930,8 @@
 const std::map<DbusValue, IpmiValue> dbusToIpmi = {
     {RestorePolicy::Policy::AlwaysOff, 0x00},
     {RestorePolicy::Policy::Restore, 0x01},
-    {RestorePolicy::Policy::AlwaysOn, 0x02}};
+    {RestorePolicy::Policy::AlwaysOn, 0x02},
+    {RestorePolicy::Policy::None, 0x03}};
 
 static constexpr uint8_t noChange = 0x03;
 static constexpr uint8_t allSupport = 0x01 | 0x02 | 0x04;