ARM: dts: Add binding documentation for AXP20x pmic usb power supply

AXP20x USB power supply

Required Properties:

-compatible: "x-powers,axp202-usb-power-supply"

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";

	};

};