iommu/mediatek: Add iova_region structure

Add a new structure for the iova_region. Each a region will be a
independent iommu domain.

For the previous SoC, there is single iova region(0~4G). For the SoC
that need support multi-domains, there will be several regions.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-27-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>