Remove the "struct mtk_smi_iommu" to simplify the code since it has only
one item in it right now.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>