iommu/arm-smmu: Support v7s context format

Fill in the last bits of machinery required to drive a stage 1 context
bank in v7 short descriptor format. By default we'll prefer to use it
only when the CPUs are also using the same format, such that we're
guaranteed that everything will be strictly 32-bit.

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