iommu: Add a capability for flush queue support

Passing a special type to domain_alloc to indirectly query whether flush
queues are a worthwhile optimisation with the given driver is a bit
clunky, and looking increasingly anachronistic. Let's put that into an
explicit capability instead.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Jerry Snitselaar <jsnitsel@redhat.com> # amd, intel, smmu-v3
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/f0086a93dbccb92622e1ace775846d81c1c4b174.1683233867.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>