powerpc/kexec: Add ifdef CONFIG_PPC_STD_MMU_64 to PPC64 code

powerpc/kexec: Add ifdef CONFIG_PPC_STD_MMU_64 to PPC64 code

powerpc/kexec: Add ifdef CONFIG_PPC_STD_MMU_64 to PPC64 code

This patch introduces PPC64 specific #ifdef bits from the upstream
commit: b3df895aebe091b1657a42a8c859bd49fc96646b.

Reported-and-tested-by: dann frazier <dannf@dannf.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
cc: Anton Blanchard <anton@samba.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>