iommu/arm-smmu: Add configuration implementation hook

Probing the ID registers and setting up the SMMU configuration is an
area where overrides and workarounds may well be needed. Indeed, the
Cavium workaround detection lives there at the moment, so let's break
that out.

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