iommu/arm-smmu: Print message when Cavium erratum 27704 was detected

Firmware is responsible for properly enabling smmu workarounds. Print
a message for better diagnostics when Cavium erratum 27704 was
detected.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>