iommu/vt-d: Add support for static identity domain

iommu/vt-d: Add support for static identity domain

Software determines VT-d hardware support for passthrough translation by
inspecting the capability register. If passthrough translation is not
supported, the device is instructed to use DMA domain for its default
domain.

Add a global static identity domain with guaranteed attach semantics for
IOMMUs that support passthrough translation mode.

The global static identity domain is a dummy domain without corresponding
dmar_domain structure. Consequently, the device's info->domain will be
NULL with the identity domain is attached. Refactor the code accordingly.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240809055431.36513-7-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>