arm64: dma-mapping: Respect device coherency in IOMMU mapper

Coherent devices can make use of the CPU cache, so they should get
coherent IOMMU mappings and should have their cache maintenance
operations skipped.  Implement this for the vanilla arm64 IOMMU mapper.

Change-Id: Iab6715a071a5fa5556934d7771a6d07d678e9703
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>