PWM controlled regulators

Required properties:
- compatible		  : Must be "regulator-pwm".
- pwms			  : Must be pwm-list specified by pwm binding pwm.txt
- regulator-min-microvolt : Must be smallest voltage at pwm duty cycle 0%
- regulator-max-microvolt : Must be largest voltage at pwm duty cycle 100%
			    (must be above min; if equal, then use a fixed
			    regulator)

Optional properties:
- regulator-n-voltages	: number of voltages, if specified implies evenly
			  distributed duty cycle levels; must be at least 2
- voltage-time-sel	: time in microseconds of voltage transition between
                          voltage levels (specified if regulator settling time
                          is independent of voltage change; for regulators with
                          fixed slew-rate use regulator-ramp-delay property)

Control GPIOs specified according to gpio binding in gpio.txt
- enable-gpio		: GPIO to use to enable/disable the regulator
- idle-gpio		: GPIO to use to enter/exit regulator idle mode
- standby-gpio		: GPIO to use to enter/exit regulator standby mode

Any property defined as part of the pwm user binding in pwm.txt.
Any property defined as part of the core regulator binding in regulator.txt.

Example:

	pwm: pwm-controller {
		#pwm-cells = <2>;
	};

	abc_pwm_reg: pwm-regulator {
		compatible = "regulator-pwm";
		pwms = <&pwm 0 3000>;
		regulator-min-microvolt = <1000000>;
		regulator-max-microvolt = <2500000>;

		regulator-name = "vdd-abc-pwm-reg";
		regulator-enable-ramp-delay = <700>;
		regulator-boot-on;

		regulator-n-voltages = <64>;
		voltage-time-sel = <50>;
		enable-gpio = <&gpio0 21 0x0>;
		standby-gpio = <&gpio0 22 0x0>;
	};
