MAINTAINERS: update gpio-zynq.yaml reference

Changeset 45ca1607 ("dt-bindings: gpio: zynq: convert bindings to YAML")
renamed: Documentation/devicetree/bindings/gpio/gpio-zynq.txt
to: Documentation/devicetree/bindings/gpio/gpio-zynq.yaml.

Update its cross-reference accordingly.

Fixes: 45ca1607

 ("dt-bindings: gpio: zynq: convert bindings to YAML")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>