iommu/mediatek: Allow building as module

The IOMMU in many SoC depends on the MM clocks and power-domain which
are device_initcall normally, thus the subsys_init here is not helpful.
This patch switches it to module_platform_driver which also allow the
driver built as module.

Correspondingly switch the config to tristate, and update the
iommu_ops's owner.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210326032337.24578-2-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>