iommu/arm-smmu-v3: Report IOMMU_CAP_ENFORCE_CACHE_COHERENCY for CANWBS

HW with CANWBS is always cache coherent and ignores PCI No Snoop requests
as well. This meets the requirement for IOMMU_CAP_ENFORCE_CACHE_COHERENCY,
so let's return it.

Implement the enforce_cache_coherency() op to reject attaching devices
that don't have CANWBS.

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Donald Dutile <ddutile@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v4-9e99b76f3518+3a8-smmuv3_nesting_jgg@nvidia.com


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