ArmPkg/ProcessorSubClassDxe: Get serial and part number from OemMiscLib

Currently, the serial and part number of a processor are filled with
fixed PCDs. However, they may be updated dynamically according to the
information being passed from a the pre-UEFI firmware during booting.
So, this patch is to support updating these string fields from
OemMiscLib if the PCDs are empty.

Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Include/Library/OemMiscLib.h

@@ -56,6 +56,8 @@ typedef enum {
   ChassisTypeType03,
   ManufacturerType03,
   SkuNumberType03,
+  ProcessorPartNumType04,
+  ProcessorSerialNumType04,
   SmbiosHiiStringFieldMax
 } OEM_MISC_SMBIOS_HII_STRING_FIELD;