iommu/arm-smmu: Abstract context bank accesses

Context bank accesses are fiddly enough to deserve a number of extra
helpers to keep the callsites looking sane, even though there are only
one or two of each.

Change-Id: I4ec6ce09eae4c41d228086b15574c3e9e816f552
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Git-commit: 19713fd4
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>