StandaloneMM: Include the newly added library class for MMU functions

StandaloneMM: Include the newly added library class for MMU functions

The MMU functions needed for StandaloneMM image are now exported
through a separate library class. Make the corresponding change in the
core's entry point inf file so that it references the correct library
class for modifying the MMU attributes.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>