ArmPkg: Validate MM communication buffer range arithmetic

Use SafeUint64Add() to validate the secure and non-secure MM
communication buffer ranges after retrieving their descriptors during
initialization. Reject descriptors whose start and size overflow before
entering the event loop.

Since the descriptors remain unchanged after initialization, keep the
simple range calculations in the address-validation functions.

Test: Built ArmStandaloneMmCoreEntryPoint for AARCH64 DEBUG_GCC.

Signed-off-by: Kajal Agarwal <kajal.agarwal@arm.com>