ARM/dma-mapping: consolidate IOMMU ops callbacks

Merge the coherent and non-coherent callbacks down to a single
implementation each, relying on the generic dev->dma_coherent
flag at the points where the difference matters.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Marc Zyngier <maz@kernel.org>