MAINTAINERS: update brcm,unimac-mdio.yaml reference

Changeset 5d1c5594 ("dt-bindings: net: brcm,unimac-mdio: convert to the json-schema")
renamed: Documentation/devicetree/bindings/net/brcm,unimac-mdio.txt
to: Documentation/devicetree/bindings/net/brcm,unimac-mdio.yaml.

Update its cross-reference accordingly.

Fixes: 5d1c5594 ("dt-bindings: net: brcm,unimac-mdio: convert to the json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/16b1368230352f3ec136d98766090a2161f84f9f.1634630486.git.mchehab+huawei@kernel.org