iommu/arm-smmu: Add framework for implementation defined features

The arm smmu architecture spec allows hw implemenations to define
additional registers and behaviors.

Use function pointers to separate this code from the upstream
implementation for better organization.

Change-Id: I26589af0ca38b557254c6db92c55960dbc49c3ec
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>