Documentation: dt: Add binding info for jz4740-musb driver

This commit adds documentation for the device-tree bindings of the
jz4740-musb driver, which provides support for the USB gadget mode
of the JZ4740 and similar SoCs from Ingenic.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/ingenic,jz4740-musb.txt

+Ingenic JZ4740 MUSB driver
+
+Required properties:
+
+- compatible: Must be "ingenic,jz4740-musb"
+- reg: Address range of the UDC register set
+- interrupts: IRQ number related to the UDC hardware
+- interrupt-names: must be "mc"
+- clocks: phandle to the "udc" clock
+- clock-names: must be "udc"
+
+Example:
+
+udc: usb@13040000 {
+	compatible = "ingenic,jz4740-musb";
+	reg = <0x13040000 0x10000>;
+
+	interrupt-parent = <&intc>;
+	interrupts = <24>;
+	interrupt-names = "mc";
+
+	clocks = <&cgu JZ4740_CLK_UDC>;
+	clock-names = "udc";
+};