MAINTAINERS: update mtk-sd.yaml reference

Changeset c626695e ("dt-bindings: mmc: Convert mtk-sd to json-schema")
renamed: Documentation/devicetree/bindings/mmc/mtk-sd.txt
to: Documentation/devicetree/bindings/mmc/mtk-sd.yaml.

Update its cross-reference accordingly.

Fixes: c626695e ("dt-bindings: mmc: Convert mtk-sd to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>