powerpc/dma: Remove dev->archdata.iommu_domain

There are no users left, so remove the pointer and save some memory.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200625130836.1916-14-joro@8bytes.org