Rockchip RK3328 GRF (General Register Files) GPIO controller.

In Rockchip RK3328, the output only GPIO_MUTE pin, originally for codec mute
control, can also be used for general purpose. It is manipulated by the
GRF_SOC_CON10 register in GRF. Aside from the GPIO_MUTE pin, the HDMI pins can
also be set in the same way.

Currently this GPIO controller only supports the mute pin. If needed in the
future, the HDMI pins support can also be added.

Required properties:
- compatible: Should contain "rockchip,rk3328-grf-gpio".
- gpio-controller: Marks the device node as a gpio controller.
- #gpio-cells: Should be 2. The first cell is the pin number and
  the second cell is used to specify the gpio polarity:
    0 = Active high,
    1 = Active low.

Example:

	grf: syscon@ff100000 {
		compatible = "rockchip,rk3328-grf", "syscon", "simple-mfd";

		grf_gpio: grf-gpio {
			compatible = "rockchip,rk3328-grf-gpio";
			gpio-controller;
			#gpio-cells = <2>;
		};
	};

Note: The grf_gpio node should be declared as the child of the GRF (General
Register File) node. The GPIO_MUTE pin is referred to as <&grf_gpio 0>.