Doc: usb: ci-hdrc-usb2: add itc-setting at binding doc

It is used to configure the ITC (in register USBCMD) value
for kinds of applications.
Signed-off-by: Peter Chen <peter.chen@freescale.com>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

@@ -30,6 +30,8 @@ Optional properties:
   argument that indicate usb controller index
 - disable-over-current: (FSL only) disable over current detect
 - external-vbus-divider: (FSL only) enables off-chip resistor divider for Vbus
+- itc-setting: interrupt threshold control register control, the setting
+  should be aligned with ITC bits at register USBCMD.
 
 Example:
 
@@ -41,4 +43,5 @@ Example:
 		phys = <&usb_phy0>;
 		phy-names = "usb-phy";
 		vbus-supply = <&reg_usb0_vbus>;
+		gadget-itc-setting = <0x4>; /* 4 micro-frames */
 	};