iommu/arm-smmu: Abstract GR1 accesses

Introduce some register access abstractions which we will later use to
encapsulate various quirks. GR1 is the easiest page to start with.

Change-Id: I2dfb3a12ebcb705874f75799ec6f1997abf488ff
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Git-commit: aadbf214
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>