mfd: sun6i-prcm: Document DT bindings

Document DT bindings of the PRCM (Power/Reset/Clock Management) unit.
Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/sun6i-prcm.txt

+* 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>;
+		};
+	};