iommu: add IOMMU_PRIV attribute

Add the IOMMU_PRIV attribute, which is used to indicate privileged
mappings.

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