iommu/mediatek: Make use of iommu_device_register interface

Register individual Mediatek IOMMUs to the iommu core and
add sysfs entries.

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>