iommu/arm-smmu: Convert ThunderX workaround to new method

With a framework for implementation-specific funtionality in place, the
currently-FDT-dependent ThunderX workaround gets to be the first user.

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>