iommu/mtk_iommu: Add an IOMMU_IDENTITIY_DOMAIN

This brings back the ops->detach_dev() code that commit
1b932ced ("iommu: Remove detach_dev callbacks") deleted and turns it
into an IDENTITY domain.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/18-v8-81230027b2fa+9d-iommu_all_defdom_jgg@nvidia.com


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