iommu/mediatek-v1: Allow building as module

This patch only adds support for building the IOMMU-v1 driver as module.
Correspondingly switch the config to tristate and update the iommu_ops's
owner to THIS_MODULE.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210326032337.24578-1-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>