Spreadtrum SC2721 Voltage regulators

The SC2721 integrates low-voltage and low quiescent current DCDC/LDO.
18 LDO and 4 DCDCs are designed for external use.

Required properties:
 - compatible: should be "sprd,sc2721-regulator" or "sprd,sc2731-regulator".

List of regulators provided by this controller. It is named according to
its regulator type, DCDC_<name> and LDO_<name>. The definition for each
of these nodes is defined using the standard binding for regulators at
Documentation/devicetree/bindings/regulator/regulator.txt.

Example:
	regulators {
		compatible = "sprd,sc2721-regulator";

		vddarm0: DCDC_CPU0 {
			regulator-name = "vddarm0";
			regulator-min-microvolt = <400000>;
			regulator-max-microvolt = <1996875>;
			regulator-ramp-delay = <25000>;
			regulator-always-on;
		};

		vddcama0: LDO_CAMA0 {
			regulator-name = "vddcama0";
			regulator-min-microvolt = <1200000>;
			regulator-max-microvolt = <3750000>;
			regulator-enable-ramp-delay = <100>;
		};
		...
	};
