IntelFsp2Pkg: Improvement of supporting null UPD pointer in FSP-T

IntelFsp2Pkg: Improvement of supporting null UPD pointer in FSP-T

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=4114

1.Use xmm5 slot 1 and xmm6 slot 3 to save ucode status and UPD pointer
  respectively in TempRamInitApi in IA32 FspSecCoreT.
2.Correct inappropriate description in the return value of
  AsmGetFspInfoHeader.
3.Replace hardcoded offset value 0x1C with FSP_HEADER_IMGBASE_OFFSET in
  FspHeler.nasm.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Ashraf Ali S <ashraf.ali.s@intel.com>
Cc: Chinni B Duggapu <chinni.b.duggapu@intel.com>
Signed-off-by: Ted Kuo <ted.kuo@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>