pwm: sunxi: document OF bindings

This is the documentation for the Allwinner SoCs PWM bindings.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-sun4i.txt

+Allwinner sun4i and sun7i SoC PWM controller
+
+Required properties:
+  - compatible: should be one of:
+    - "allwinner,sun4i-a10-pwm"
+    - "allwinner,sun7i-a20-pwm"
+  - reg: physical base address and length of the controller's registers
+  - #pwm-cells: should be 3. See pwm.txt in this directory for a description of
+    the cells format.
+  - clocks: From common clock binding, handle to the parent clock.
+
+Example:
+
+	pwm: pwm@01c20e00 {
+		compatible = "allwinner,sun7i-a20-pwm";
+		reg = <0x01c20e00 0xc>;
+		clocks = <&osc24M>;
+		#pwm-cells = <3>;
+		status = "disabled";
+	};