iommu/exynos: Implement an IDENTITY domain

What exynos calls exynos_iommu_detach_device is actually putting the iommu
into identity mode.

Move to the new core support for ARM_DMA_USE_IOMMU by defining
ops->identity_domain.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/10-v8-81230027b2fa+9d-iommu_all_defdom_jgg@nvidia.com


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