regulator: pwm-regulator: add devicetree bindings for pwm regulator

Document the pwm regulator
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

+pwm regulator bindings
+
+Required properties:
+- compatible: Should be "pwm-regulator"
+- pwms: OF device-tree PWM specification (see PWM binding pwm.txt)
+- voltage-table: voltage and duty table, include 2 members in each set of
+  brackets, first one is voltage(unit: uv), the next is duty(unit: percent)
+
+Any property defined as part of the core regulator binding defined in
+regulator.txt can also be used.
+
+Example:
+	pwm_regulator {
+		compatible = "pwm-regulator;
+		pwms = <&pwm1 0 8448 0>;
+
+		voltage-table = <1114000 0>,
+				<1095000 10>,
+				<1076000 20>,
+				<1056000 30>,
+				<1036000 40>,
+				<1016000 50>;
+
+		regulator-min-microvolt = <1016000>;
+		regulator-max-microvolt = <1114000>;
+		regulator-name = "vdd_logic";
+	};