iommu: Hide ops.domain_alloc behind CONFIG_FSL_PAMU

fsl_pamu is the last user of domain_alloc(), and it is using it to create
something weird that doesn't really fit into the iommu subsystem
architecture. It is a not a paging domain since it doesn't have any
map/unmap ops. It may be some special kind of identity domain.

For now just leave it as is. Wrap it's definition in CONFIG_FSL_PAMU to
discourage any new drivers from attempting to use it.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/5-v4-ff5fb6b03bd1+288-iommu_virtio_domains_jgg@nvidia.com


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