iommu/arm-smmu: Add implementation infrastructure

Add some nascent infrastructure for handling implementation-specific
details outside the flow of the architectural code. This will allow us
to keep mutually-incompatible vendor-specific hooks in their own files
where the respective interested parties can maintain them with minimal
chance of conflicts. As somewhat of a template, we'll start with a
general place to collect the relatively trivial existing quirks.

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