iommu/dart: Use static global identity domains

Move to the new static global for identity domains. Move the identity
specific code to apple_dart_attach_dev_identity().

Reviewed-by: Janne Grunau <j@jannau.net>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Sven Peter <sven@svenpeter.dev>
Link: https://lore.kernel.org/r/5-v2-bff223cf6409+282-dart_paging_jgg@nvidia.com


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