iommu: Add domain_alloc_identity()

virtio-iommu has a mode where the IDENTITY domain is actually a paging
domain with an identity mapping covering some of the system address
space manually created.

To support this add a new domain_alloc_identity() op that accepts
the struct device so that virtio can allocate and fully finalize a
paging domain to return.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/2-v4-ff5fb6b03bd1+288-iommu_virtio_domains_jgg@nvidia.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>