dt-bindings: arm: mediatek: mmsys: update mediatek,mmsys.yaml reference

Changeset cba3c40d ("dt-bindings: arm: mediatek: mmsys: convert to YAML format")
renamed: Documentation/devicetree/bindings/arm/mediatek/mediatek,mmsys.txt
to: Documentation/devicetree/bindings/arm/mediatek/mediatek,mmsys.yaml.

Update its cross-reference accordingly.

Fixes: cba3c40d ("dt-bindings: arm: mediatek: mmsys: convert to YAML format")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/a87eb079a73e8ab41cdf6e40e80b1d1f868da6bd.1631785820.git.mchehab+huawei@kernel.org