iommu/arm-smmu: Implement of_xlate() for SMMUv3

Now that we can properly describe the mapping between PCI RIDs and
stream IDs via "iommu-map", and have it fed it to the driver
automatically via of_xlate(), rework the SMMUv3 driver to benefit from
that, and get rid of the current misuse of the "iommus" binding.

Since having of_xlate wired up means that masters will now be given the
appropriate DMA ops, we also need to make sure that default domains work
properly. This necessitates dispensing with the "whole group at a time"
notion for attaching to a domain, as devices which share a group get
attached to the group's default domain one by one as they are initially
probed.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>