iommu/mediatek: Add mt2712 IOMMU support

The M4U IP blocks in mt2712 is MTK's generation2 M4U which use the
ARM Short-descriptor like mt8173, and most of the HW registers are
the same.

The difference is that there are 2 M4U HWs in mt2712 while there's
only one in mt8173. The purpose of 2 M4U HWs is for balance the
bandwidth.

Normally if there are 2 M4U HWs, there should be 2 iommu domains,
each M4U has a iommu domain.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>