iommu/arm-smmu: Rework cb_base handling

To keep register-access quirks manageable, we want to structure things
to avoid needing too many individual overrides. It seems fairly clean to
have a single interface which handles both global and context registers
in terms of the architectural pages, so the first preparatory step is to
rework cb_base into a page number rather than an absolute address.

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