[S390] Optional ZONE_DMA for s390.

Disable ZONE_DMA on 31-bit. All memory is addressable by all
devices and we do not need any special memory pool.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>