mfd: sun6i-prcm: Document DT bindings

* Allwinner PRCM (Power/Reset/Clock Management) Multi-Functional Device

PRCM is an MFD device exposing several Power Management related devices

(like clks and reset controllers).

Required properties:

 - compatible: "allwinner,sun6i-a31-prcm"

 - reg: The PRCM registers range

The prcm node may contain several subdevices definitions:

 - see Documentation/devicetree/clk/sunxi.txt for clock devices

 - see Documentation/devicetree/reset/allwinner,sunxi-clock-reset.txt for reset

   controller devices

Example:

	prcm: prcm@01f01400 {

		compatible = "allwinner,sun6i-a31-prcm";

		reg = <0x01f01400 0x200>;

		/* Put subdevices here */

		ar100: ar100_clk {

			compatible = "allwinner,sun6i-a31-ar100-clk";

			#clock-cells = <0>;

			clocks = <&osc32k>, <&osc24M>, <&pll6>, <&pll6>;

		};

		ahb0: ahb0_clk {

			compatible = "fixed-factor-clock";

			#clock-cells = <0>;

			clock-div = <1>;

			clock-mult = <1>;

			clocks = <&ar100_div>;

			clock-output-names = "ahb0";

		};

		apb0: apb0_clk {

			compatible = "allwinner,sun6i-a31-apb0-clk";

			#clock-cells = <0>;

			clocks = <&ahb0>;

			clock-output-names = "apb0";

		};

		apb0_gates: apb0_gates_clk {

			compatible = "allwinner,sun6i-a31-apb0-gates-clk";

			#clock-cells = <1>;

			clocks = <&apb0>;

			clock-output-names = "apb0_pio", "apb0_ir",

					"apb0_timer01", "apb0_p2wi",

					"apb0_uart", "apb0_1wire",

					"apb0_i2c";

		};

		apb0_rst: apb0_rst {

			compatible = "allwinner,sun6i-a31-clock-reset";

			#reset-cells = <1>;

		};

	};