MAINTAINERS: update gemini.yaml reference

Changeset 7da6ebf5 ("dt-bindings: arm: Convert Gemini boards to YAML")
renamed: Documentation/devicetree/bindings/arm/gemini.txt
to: Documentation/devicetree/bindings/arm/gemini.yaml.

Update its cross-reference accordingly.

Fixes: 7da6ebf5

 ("dt-bindings: arm: Convert Gemini boards to YAML")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/dd52c97597a073a5830ecf115d4a6516eebc6d6a.1634630486.git.mchehab+huawei@kernel.org