iommu/mediatek: Adjust the structure

Add "struct mtk_iommu_data *" in the "struct mtk_iommu_domain",
reduce the call mtk_iommu_get_m4u_data().
No functional change.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-24-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>