MdeModulePkg/PeiCore: honour minimal runtime allocation granularity

MdeModulePkg/PeiCore: honour minimal runtime allocation granularity

Architectures such as AArch64 may run the OS with 16 KB or 64 KB sized
pages, and for this reason, the UEFI spec mandates a minimal allocation
granularity of 64 KB for regions that may require different memory
attributes at OS runtime.

So make PeiCore's implementation of AllocatePages () take this into
account as well.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>