iommu/arm-smmu: Abstract GR0 accesses

Clean up the remaining accesses to GR0 registers, so that everything is
now neatly abstracted. This folds up the Non-Secure alias quirk as the
first step towards moving it out of the way entirely. Although GR0 does
technically contain some 64-bit registers (sGFAR and the weird SMMUv2
HYPC and MONC stuff), they're not ones we have any need to access.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>