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

The FIELD_{GET,PREP} accessors provided by linux/bitfield.h allow us to
define multi-bit register fields solely in terms of their bit positions
via GENMASK(), without needing explicit *_SHIFT and *_MASK definitions.
As well as the immediate reduction in lines of code, this avoids the
awkwardness of values sometimes being pre-shifted and sometimes not,
which means we can factor out some common values like memory attributes.
Furthermore, it also makes it trivial to verify the definitions against
the architecture spec, on which note let's also fix up a few field names
to properly match the current release (IHI0070B).

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