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

Add binding documentation for the usb power supply part of the AXP20x pmic.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power_supply/axp20x_usb_power.txt

+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";
+	};
+};