iommu/io-pgtable-arm-v7s: Add support for the IOMMU_PRIV flag

The short-descriptor format also allows privileged-only mappings, so
let's wire it up.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>