MdeModulePkg/Dxe/Gcd: disregard memory above MAX_ALLOC_ADDRESS

MdeModulePkg/Dxe/Gcd: disregard memory above MAX_ALLOC_ADDRESS

Update the GCD memory map initialization code so it disregards
memory that is not addressable by the CPU at boot time. This
only affects the first memory descriptor that is added, other
memory descriptors are permitted that describe memory ranges
that may be accessible to the CPU itself only when executing
under the OS.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>