GPIO controlled regulators

Required properties:
- compatible		: Must be "regulator-gpio".
- states		: Selection of available voltages and GPIO configs.
                          if there are no states, then use a fixed regulator

Optional properties:
- enable-gpio		: GPIO to use to enable/disable the regulator.
- gpios			: GPIO group used to control voltage.
- startup-delay-us	: Startup time in microseconds.
- enable-active-high	: Polarity of GPIO is active high (default is low).

Any property defined as part of the core regulator binding defined in
regulator.txt can also be used.

Example:

	mmciv: gpio-regulator {
		compatible = "regulator-gpio";

		regulator-name = "mmci-gpio-supply";
		regulator-min-microvolt = <1800000>;
		regulator-max-microvolt = <2600000>;
		regulator-boot-on;

		enable-gpio = <&gpio0 23 0x4>;
		gpios = <&gpio0 24 0x4
			 &gpio0 25 0x4>;
		states = <1800000 0x3
			  2200000 0x2
			  2600000 0x1
			  2900000 0x0>;

		startup-delay-us = <100000>;
		enable-active-high;
	};