MdePkg/PiSmmCis.h: SMM register protocol notify function clarify

MdePkg/PiSmmCis.h: SMM register protocol notify function clarify

PI 1.4 clarified SMM register protocol notify function return status as below:
EFI_SUCCESS           Successfully returned the registration record that has
                      been added or unhooked
EFI_INVALID_PARAMETER Protocol is NULL or Registration is NULL

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17348 6f19259b-4bc3-4df7-8a09-765794883524