IntelFsp2Pkg: Add FunctionParametePtr to FspGlobalData.

IntelFsp2Pkg: Add FunctionParametePtr to FspGlobalData.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2726

When FSP switching stack and calling bootloader functions,
the function parameter in stack may not be accessible easily.
We can store the function parameter pointer to FspGlobalData
and retrieve it after stack switched.

Also need to add Loader2PeiSwitchStack () to header file
as public function for platform FSP code to consume.

Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>