iommu/arm-smmu: Track context bank state

Echoing what we do for Stream Map Entries, maintain a software shadow
state for context bank configuration. With this in place, we are mere
moments away from blissfully easy suspend/resume support.

Reviewed-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[will: fix sparse warning by only clearing .cfg during domain destruction]
Signed-off-by: Will Deacon <will.deacon@arm.com>