iommu/mediatek-v1: Avoid build fail when build as module

When this driver build as module, It build fail like:

ERROR: modpost: "of_phandle_iterator_args"
[drivers/iommu/mtk_iommu_v1.ko] undefined!

This patch remove this interface to avoid this build fail.

Reported-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210412064843.11614-1-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>