iommu/mediatek: Do not call of_node_put in mtk_iommu_of_xlate

The device_node will be released in of_iommu_configure, it may be double
released if call of_node_put in mtk_iommu_of_xlate.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>