MdePkg/ProcessorBind.h AARCH64: limit MAX_ADDRESS to 48 bits

AArch64 supports the use of more than 48 bits for physical and/or
virtual addressing, but only if the page size is set to 64 KB,
which is not supported by UEFI. So redefine MAX_ADDRESS to cover
only 48 address bits.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>