This allows to handle domains differently based on their
type in the future. An IOMMU driver can implement certain
optimizations for DMA-API domains for example.

The domain types can be extended later and some of the
existing domain attributes can be migrated to become domain
flags.
Tested-by: NThierry Reding <treding@nvidia.com>
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Reviewed-by: NAlex Williamson <alex.williamson@redhat.com>
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>