dt-bindings: gpio: fix gpio-hog example

Even if this is no yaml yet at least fix the example to be compliant to
later schema as e.g. found in gpio-pca95xx.yaml, fairchild,74hc595.yaml
and gpio/fsl-imx-gpio.yaml.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>