Fixed Voltage regulator

Binding:
The binding is done by the property "compatible" - this is different, than for
binding by the node prefix (doc/device-tree-bindings/regulator/regulator.txt).

Required properties:
- compatible: "regulator-fixed"
- regulator-name: this is required by the regulator uclass

Optional properties:
- gpio: GPIO to use for enable control
- startup-delay-us: startup time in microseconds
- regulator constraints (binding info: regulator.txt)

Other kernel-style properties, are currently not used.

Note:
For the regulator constraints, driver expects that:
- regulator-min-microvolt is equal to regulator-max-microvolt
- regulator-min-microamp is equal to regulator-max-microamp

Example:
fixed_regulator@0 {
	/* Mandatory */
	compatible = "regulator-fixed";
	regulator-name = "LED_3.3V";

	/* Optional: */
	gpio = <&gpc1 0 GPIO_ACTIVE_LOW>;

	/* Optional for regulator uclass */
	regulator-min-microvolt = <3300000>;
	regulator-max-microvolt = <3300000>;
	regulator-min-microamp = <15000>;
	regulator-max-microamp = <15000>;
	regulator-always-on;
	regulator-boot-on;
};