MdePkg PeiServicesLib: Add PeiServicesFreePages
Add PeiServicesFreePages and update
comments for PeiServicesAllocatePages.
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>