iommu/arm-smmu: Differentiate specific implementations

As the inevitable reality of implementation-specific errata workarounds
begin to accrue alongside our integration quirk handling, it's about
time the driver had a decent way of keeping track. Extend the per-SMMU
data so we can identify specific implementations in an efficient and
firmware-agnostic manner.

Acked-by: Tirumalesh Chalamarla <tchalamarla@caviumnetworks.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>