MdeModulePkg/DxeCore: Fixed build error.

MdeModulePkg/DxeCore: Fixed build error.

ARM toolchain raises the build error: "enumerated type mixed with
another type".

To fix the issue, typecase can be used like below.
 -    return EfiMaxMemoryType + 1;
 +    return (EFI_MEMORY_TYPE)(EfiMaxMemoryType + 1);

But to eliminate the confusion, update the return type of
GetProfileMemoryIndex() from EFI_MEMORY_TYPE to UINTN.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17535 6f19259b-4bc3-4df7-8a09-765794883524