iommu/arm-smmu: Set PRIVCFG in stage 1 STEs

Implement the SMMUv3 equivalent of d346180e ("iommu/arm-smmu: Treat
all device transactions as unprivileged"), so that once again those
pesky DMA controllers with their privileged instruction fetches don't
unexpectedly fault in stage 1 domains due to VMSAv8 rules.

Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>