MdePkg: Add helper function to detect RME

Add helper function to check if the Realm Management
Extension (RME) is implemented by the hardware.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Leif Lindholm <leif.lindholm@oss.qualcomm.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>