Support unhook notification function for Smst->SmmRegisterProtocolNotify() per PI specification.

Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@14823 6f19259b-4bc3-4df7-8a09-765794883524