Fixed Voltage regulators

Required properties:
- compatible: Must be "regulator-fixed";
- regulator-name: Defined in regulator.txt as optional, but required here.

Optional properties:
- gpio: gpio to use for enable control
- startup-delay-us: startup time in microseconds
- enable-active-high: Polarity of GPIO is Active high
If this property is missing, the default assumed is Active low.
- gpio-open-drain: GPIO is open drain type.
  If this property is missing then default assumption is false.
-vin-supply: Input supply name.

Any property defined as part of the core regulator
binding, defined in regulator.txt, can also be used.
However a fixed voltage regulator is expected to have the
regulator-min-microvolt and regulator-max-microvolt
to be the same.

Example:

	abc: fixedregulator@0 {
		compatible = "regulator-fixed";
		regulator-name = "fixed-supply";
		regulator-min-microvolt = <1800000>;
		regulator-max-microvolt = <1800000>;
		gpio = <&gpio1 16 0>;
		startup-delay-us = <70000>;
		enable-active-high;
		regulator-boot-on;
		gpio-open-drain;
		vin-supply = <&parent_reg>;
	};