iommu/mtk_iommu_v1: Implement an IDENTITY domain

iommu/mtk_iommu_v1: Implement an IDENTITY domain

What mtk does during mtk_iommu_v1_set_platform_dma() is actually putting
the iommu into identity mode. Make this available as a proper IDENTITY
domain.

The mtk_iommu_v1_def_domain_type() from
commit 8bbe13f52cb7 ("iommu/mediatek-v1: Add def_domain_type") explains
this was needed to allow probe_finalize() to be called, but now the
IDENTITY domain will do the same job so change the returned
def_domain_type.

mkt_v1 is the only driver that returns IOMMU_DOMAIN_UNMANAGED from
def_domain_type().  This allows the next patch to enforce an IDENTITY
domain policy for this driver.

Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/7-v8-81230027b2fa+9d-iommu_all_defdom_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>