dt: Binding documentation for isp1704 charger

Add devicetree binding documentation for isp1704-charger.
Signed-off-by: NSebastian Reichel <sre@debian.org>
Signed-off-by: NAnton Vorontsov <anton@enomsg.org>

Documentation/devicetree/bindings/power/isp1704.txt

+Binding for NXP ISP1704 USB Charger Detection
+
+Required properties:
+- compatible: Should contain one of the following:
+ * "nxp,isp1704"
+- nxp,enable-gpio: Should contain a phandle + gpio-specifier
+  to the GPIO pin connected to the chip's enable pin.
+- usb-phy: Should contain a phandle to the USB PHY
+  the ISP1704 is connected to.
+
+Example:
+
+isp1704 {
+	compatible = "nxp,isp1704";
+	nxp,enable-gpio = <&gpio3 3 GPIO_ACTIVE_LOW>;
+	usb-phy = <&usb2_phy>;
+};