iommu/arm-smmu: Decouple context format from kernel config

The way the driver currently forces an AArch32 or AArch64 context format
based on the kernel config and SMMU architecture version is suboptimal,
in that it makes it very hard to support oddball mix-and-match cases
like the SMMUv1 64KB supplement, or situations where the reduced table
depth of an AArch32 short descriptor context may be desirable under an
AArch64 kernel. It also only happens to work on current implementations
which do support all the relevant formats.

Introduce an explicit notion of context format, so we can manage that
independently and get rid of the inflexible #ifdeffery.

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