IntelFsp2Pkg: Add function to get bootloader stack pointer

IntelFsp2Pkg: Add function to get bootloader stack pointer

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1391

FSP on-going enhancement for stack utilization required
to know bootloader stack pointer and this pointer can be
retrieved by first input parameter address when FSP-M
entry API invoked by bootloader.

Test: Verified on internal platform and booting successfully

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>