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.

Change-Id: Ie1ee37e9250a8295bc07cb78d879b98452f50920
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Git-commit: fc058d37
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: resolved merge conflics and build errors]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>