arm: Remove dev->archdata.iommu pointer

There are no users left, all drivers have been converted to use the
per-device private pointer offered by IOMMU core.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200625130836.1916-12-joro@8bytes.org