dt-bindings: iommu: mediatek: update mediatek,iommu.yaml references

Changeset bca28426 ("dt-bindings: iommu: mediatek: Convert IOMMU to DT schema")
renamed: Documentation/devicetree/bindings/iommu/mediatek,iommu.txt
to: Documentation/devicetree/bindings/iommu/mediatek,iommu.yaml.

Update the cross-references accordingly.

Fixes: bca28426 ("dt-bindings: iommu: mediatek: Convert IOMMU to DT schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>