GPIO controlled regulators

Required properties:
- compatible		: Must be "regulator-gpio".
- regulator-name	: Defined in regulator.txt as optional, but required
			  here.
- gpios			: Array of one or more GPIO pins used to select the
			  regulator voltage/current listed in "states".
- states		: Selection of available voltages/currents provided by
			  this regulator and matching GPIO configurations to
			  achieve them. If there are no states in the "states"
			  array, use a fixed regulator instead.

Optional properties:
- enable-gpios		: GPIO used to enable/disable the regulator.
			  Warning, the GPIO phandle flags are ignored and the
			  GPIO polarity is controlled solely by the presence
			  of "enable-active-high" DT property. This is due to
			  compatibility with old DTs.
- enable-active-high	: Polarity of "enable-gpio" GPIO is active HIGH.
			  Default is active LOW.
- gpios-states		: On operating systems, that don't support reading back
			  gpio values in output mode (most notably linux), this
			  array provides the state of GPIO pins set when
			  requesting them from the gpio controller. Systems,
			  that are capable of preserving state when requesting
			  the lines, are free to ignore this property.
			  0: LOW, 1: HIGH. Default is LOW if nothing else
			  is specified.
- startup-delay-us	: Startup time in microseconds.
- regulator-type	: Specifies what is being regulated, must be either
			  "voltage" or "current", defaults to voltage.

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-gpios = <&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;
	};
