MAINTAINERS: update brcm,bcm-v3d.yaml reference

Changeset cb18a797 ("dt-bindings: gpu: Convert v3d to json-schema")
renamed: Documentation/devicetree/bindings/gpu/brcm,bcm-v3d.txt
to: Documentation/devicetree/bindings/gpu/brcm,bcm-v3d.yaml.

Update its cross-reference accordingly.

Fixes: cb18a797 ("dt-bindings: gpu: Convert v3d to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>