x86, mm: Wrap ZONE_DMA32 with CONFIG_ZONE_DMA32

In preparation for unifying 32-bit and 64-bit zone_sizes_init()
make sure ZONE_DMA32 is wrapped in CONFIG_ZONE_DMA32.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Arun Sharma <asharma@fb.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1320155902-10424-4-git-send-email-penberg@kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>