arm64/dma-mapping: Implement DMA_ATTR_PRIVILEGED

The newly added DMA_ATTR_PRIVILEGED is useful for creating mappings that
are only accessible to privileged DMA engines.  Implement it in
dma-iommu.c so that the ARM64 DMA IOMMU mapper can make use of it.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>