iommu/mediatek: Do no use dev->archdata.iommu

The iommu private pointer is already used in the Mediatek IOMMU v1
driver, so move the dma_iommu_mapping pointer into 'struct
mtk_iommu_data' and do not use dev->archdata.iommu anymore.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200625130836.1916-9-joro@8bytes.org