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

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";

	};