MAINTAINERS: update ste,mcde.yaml reference

Changeset 2abb0b99 ("dt-bindings: display: mcde: Convert to YAML schema")
renamed: Documentation/devicetree/bindings/display/ste,mcde.txt
to: Documentation/devicetree/bindings/display/ste,mcde.yaml.

Update its cross-reference accordingly.

Fixes: 2abb0b99

 ("dt-bindings: display: mcde: Convert to YAML schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>