ARM/dma-mapping: move various helpers from dma-mapping.h to dma-direct.h

Move the helpers to translate to and from direct mapping DMA addresses
to dma-direct.h.  This not only is the most logical place, but the new
placement also avoids dependency loops with pending commits.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>