iommu/fsl_pamu: Implement a PLATFORM domain

This driver is nonsensical. To not block migrating the core API away from
NULL default_domains give it a hacky of a PLATFORM domain that keeps it
working exactly as it always did.

Leave some comments around to warn away any future people looking at this.

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/5-v8-81230027b2fa+9d-iommu_all_defdom_jgg@nvidia.com


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