iommu/mediatek: Merge 2 M4U HWs into one iommu domain

In theory, If there are 2 M4U HWs, there should be 2 IOMMU domains.
But one IOMMU domain(4GB iova range) is enough for us currently,
It's unnecessary to maintain 2 pagetables.

Besides, This patch can simplify our consumer code largely. They don't
need map a iova range from one domain into another, They can share the
iova address easily.

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