iommu/arm-smmu-v3: Clean up table definitions

As with registers, use GENMASK and the bitfield accessors consistently
for table fields, to save some lines and ease maintenance a little. This
also catches a subtle off-by-one wherein bit 5 of CD.T0SZ was missing.

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