dt-bindings: gpio: rockchip: add a pattern for gpio hogs

Allow validating gpio-hogs defined inside the gpio controller node.

Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>