dt-bindings: gpio: realtek: Add realtek,rtd-gpio

Add the device tree bindings for the Realtek DHC(Digital Home Center)
RTD SoCs GPIO controllers.

Signed-off-by: Tzuyi Chang <tychang@realtek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>