ARM: NOMMU: Wire-up default DMA interface

The way how default DMA pool is exposed has changed and now we need to
use dedicated interface to work with it. This patch makes alloc/release
operations to use such interface. Since, default DMA pool is not
handled by generic code anymore we have to implement our own mmap
operation.

Tested-by: Andras Szemzo <sza@esh.hu>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>