iommu/mediatek: Use a struct as the platform data

Use a struct as the platform special data instead of the enumeration.
This is a prepare patch for adding mt8183 iommu support.

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