iommu/arm-smmu: Update impl quirks comment

The comment about implementation and integration quirks being
mutually-exclusive is out of date, and in fact the code is already
structured for the case it anticipates, so document that properly.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1e742177e084621f3454fbaf768325a6c215656a.1592994291.git.robin.murphy@arm.com


Signed-off-by: Will Deacon <will@kernel.org>