ARM/dma-mapping: merge IOMMU ops

The dma_sync_* operations are now the only difference between the
coherent and non-coherent IOMMU ops. Some minor tweaks to make those
safe for coherent devices with minimal overhead, and we can condense
down to a single set of DMA ops.

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