arm/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.  Adding it to the
arm dma-mapping.c so that the ARM32 DMA IOMMU mapper can make use of it.

Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>