MAINTAINERS: update cortina,gemini-ethernet.yaml reference

Changeset 208b65f7 ("dt-bindings: net: convert net/cortina,gemini-ethernet to yaml")
renamed: Documentation/devicetree/bindings/net/cortina,gemini-ethernet.txt
to: Documentation/devicetree/bindings/net/cortina,gemini-ethernet.yaml.

Update its cross-reference accordingly.

Fixes: 208b65f7

 ("dt-bindings: net: convert net/cortina,gemini-ethernet to yaml")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/d27b5d508fb757147b720bf573ce5a2e3fc5920e.1654529011.git.mchehab@kernel.org