AXP20x USB power supply

Required Properties:
-compatible: One of: "x-powers,axp202-usb-power-supply"
                     "x-powers,axp221-usb-power-supply"
                     "x-powers,axp223-usb-power-supply"

The AXP223 PMIC shares most of its behaviour with the AXP221 but has slight
variations such as the former being able to set the VBUS power supply max
current to 100mA, unlike the latter.

This node is a subnode of the axp20x PMIC.

Example:

axp209: pmic@34 {
	compatible = "x-powers,axp209";
	reg = <0x34>;
	interrupt-parent = <&nmi_intc>;
	interrupts = <0 IRQ_TYPE_LEVEL_LOW>;
	interrupt-controller;
	#interrupt-cells = <1>;

	regulators {
		x-powers,dcdc-freq = <1500>;

		vdd_cpu: dcdc2 {
			regulator-always-on;
			regulator-min-microvolt = <1000000>;
			regulator-max-microvolt = <1450000>;
			regulator-name = "vdd-cpu";
		};

		...
	};

	usb-power-supply: usb-power-supply {
		compatible = "x-powers,axp202-usb-power-supply";
	};
};