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>

IntelFsp2Pkg/Include/Library/FspSwitchStackLib.h

@@ -1,6 +1,6 @@
 /** @file
 
-  Copyright (c) 2014, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2014 - 2020, Intel Corporation. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -36,4 +36,20 @@ Pei2LoaderSwitchStack (
   VOID
   );
 
+/**
+
+  This function is equivalent to Pei2LoaderSwitchStack () but just indicates
+  the stack after switched is FSP stack.
+
+  @return ReturnKey          After switching to the saved stack,
+                             this value will be saved in eax before returning.
+
+
+**/
+UINT32
+EFIAPI
+Loader2PeiSwitchStack (
+  VOID
+  );
+
 #endif