iommu: dma-mapping-fast: 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 "fast" arm64 IOMMU mapper.

Change-Id: I30db64cae2cc4cda0baa3cdd3860dc079f22bf71
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>