MdePkg PiPeiCis.h: Add FreePages definition
MdePkg PiPeiCis.h: Add FreePages definition

Follow PI 1.6 spec to add FreePages definition and comments for
AllocatePages that will support pre permanent memory page allocation.

Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <5c6bb72efe464613af94e84c05522a361b23844a@intel.com>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2