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>