iommu/arm-smmu: Clarify MMU-500 CPRE workaround

CPRE workarounds are implicated in at least 5 MMU-500 errata, some of
which remain unfixed. The comment and warning message have proven to be
unhelpfully misleading about this scope, so reword them to get the point
across with less risk of going out of date or confusing users.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/dfa82171b5248ad7cf1f25592101a6eec36b8c9a.1728400877.git.robin.murphy@arm.com


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