IntelFsp2Pkg/FspSecCore: Add FSP-I API for SMM support.

IntelFsp2Pkg/FspSecCore: Add FSP-I API for SMM support.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3993
Add FSP-I API entry point for SMM support.
Also update 64bit API entry code to assign ApiIdx to RAX
to avoid confusion.

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Signed-off-by: Hongbin1 Zhang <hongbin1.zhang@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>