iommu/arm-smmu: Force identity domains for legacy binding

When using the legacy "mmu-masters" DT binding, we reject DMA domains
since we have no guarantee of driver probe order and thus can't rely on
client drivers getting the correct DMA ops. However, we can do better
than fall back to the old no-default-domain behaviour now, by forcing an
identity default domain instead. This also means that detaching from a
VFIO domain can actually work - that looks to have been broken for over
6 years, so clearly isn't something that legacy binding users care
about, but we may as well make the driver code make sense anyway.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/9805e4c492cb972bdcdd57999d2d001a2d8b5aab.1652171938.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>