ArmVirtPkg: QemuVirtMemInfoPeiLib: Allow PcdSystemMemorySize to be non-fixed

Platforms today may use this PCD as a dynamic PCD as that is an
allowed type in its PCD declaration. From `ArmPkg.dec`:

[PcdsFixedAtBuild.common, PcdsDynamic.common, PcdsPatchableInModule.common]
  gArmTokenSpaceGuid.PcdSystemMemorySize|0|UINT64|0x0000002A

This library causes a build error if it used as a dynamic PCD since
it places the PCD in a `[FixedPcd]` section in the INF.

Other libraries do set the PCD and depend on the dynamic PCD behavior.

Since this library accesses the PCD with `PcdGet64 ()` which is
compatible with FixedAtBuild PCDs, this change moves the PCD out an
explicit `[FixedPcd]` section to resolve the following build error:

```
  INFO -  : error 3000: Building modules from source INFs, following
                        PCD use Dynamic and FixedAtBuild access method.
                        It must be corrected to use only one access
                        method.
  INFO - 	gArmTokenSpaceGuid.PcdSystemMemorySize
```

Signed-off-by: Michael Kubacki <[email protected]>

Author: makubacki

ArmVirtPkg/Library/QemuVirtMemInfoLib/QemuVirtMemInfoPeiLib.inf

@@ -38,11 +38,16 @@
 [Guids]
   gArmVirtSystemMemorySizeGuid
 
+# MU_CHANGE [BEGIN] - Allow PcdSystemMemorySize to be referenced per allowed (i.e. DEC declaration) PCD types
+[Pcd]
+  gArmTokenSpaceGuid.PcdSystemMemorySize
+# MU_CHANGE [END] -  Allow PcdSystemMemorySize to be referenced per allowed (i.e. DEC declaration) PCD types
+
 [FixedPcd]
   gArmTokenSpaceGuid.PcdFdBaseAddress
   gArmTokenSpaceGuid.PcdFvBaseAddress
   gArmTokenSpaceGuid.PcdSystemMemoryBase
-  gArmTokenSpaceGuid.PcdSystemMemorySize
+  # gArmTokenSpaceGuid.PcdSystemMemorySize  # MU_CHANGE - Allow the PCD to be referenced per allowed PCD types
   gArmTokenSpaceGuid.PcdFdSize
   gArmTokenSpaceGuid.PcdFvSize
   gUefiOvmfPkgTokenSpaceGuid.PcdDeviceTreeInitialBaseAddress