Doc: usb: ci-hdrc-usb2: Add phy-clkgate-delay-us entry

Add an entry for the optional 'phy-clkgate-delay-us' property that is
used to describe the delay time between putting PHY into low power
mode and turning off the PHY clock.
Signed-off-by: Li Jun <jun.li@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>

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

@@ -51,6 +51,8 @@ Optional properties:
   to external connector device, which provide "USB-HOST" cable events. If one
   of the external connector devices is not required, empty <0> phandle should
   be specified.
+- phy-clkgate-delay-us: the delay time (us) between putting the PHY into
+  low power mode and gating the PHY clock.
 
 Example:
 
@@ -68,4 +70,5 @@ Example:
 		tx-burst-size-dword = <0x10>; /* 64 bytes */
 		rx-burst-size-dword = <0x10>;
 		extcon = <0>, <&usb_id>;
+		phy-clkgate-delay-us = <400>;
 	};