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

Change-Id: I3935f9f9b7526cb5c5906093131e856159b0899b
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Git-commit: 0caf5f4e
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: resolved merge conflicts/build errors]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>