iommu/arm-smmu: Convert GR0 registers to bitfields

FIELD_PREP remains a terrible name, but the overall simplification will
make further work on this stuff that much more manageable. This also
serves as an audit of the header, wherein we can impose a consistent
grouping and ordering of the offset and field definitions

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