powerpc/mm: fix undefined reference to `.__kernel_map_pages' on FSL PPC64

arch/powerpc has __kernel_map_pages implementations in mm/pgtable_32.c, and
mm/hash_utils_64.c, of which the former is built for PPC32, and the latter
for PPC64 machines with PPC_STD_MMU.  Fix arch/powerpc/Kconfig to not select
ARCH_SUPPORTS_DEBUG_PAGEALLOC when CONFIG_PPC_STD_MMU_64 isn't defined,
i.e., for 64-bit book3e builds to use the generic __kernel_map_pages()
in mm/debug-pagealloc.c.

  LD      init/built-in.o
mm/built-in.o: In function `kernel_map_pages':
include/linux/mm.h:2076: undefined reference to `.__kernel_map_pages'
include/linux/mm.h:2076: undefined reference to `.__kernel_map_pages'
include/linux/mm.h:2076: undefined reference to `.__kernel_map_pages'
Makefile:925: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>