doc: usb: chipidea: add usb wakeup enable example

Add the example for how to enable USB as system wakeup source.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/chipidea.txt

@@ -69,3 +69,24 @@ cat /sys/kernel/debug/ci_hdrc.0/registers
 ----------------------
 "On-The-Go and Embedded Host Supplement to the USB Revision 2.0 Specification
 July 27, 2012 Revision 2.0 version 1.1a"
+
+2. How to enable USB as system wakeup source
+-----------------------------------
+Below is the example for how to enable USB as system wakeup source
+at imx6 platform.
+
+2.1 Enable core's wakeup
+echo enabled > /sys/bus/platform/devices/ci_hdrc.0/power/wakeup
+2.2 Enable glue layer's wakeup
+echo enabled > /sys/bus/platform/devices/2184000.usb/power/wakeup
+2.3 Enable PHY's wakeup (optional)
+echo enabled > /sys/bus/platform/devices/20c9000.usbphy/power/wakeup
+2.4 Enable roothub's wakeup
+echo enabled > /sys/bus/usb/devices/usb1/power/wakeup
+2.5 Enable related device's wakeup
+echo enabled > /sys/bus/usb/devices/1-1/power/wakeup
+
+If the system has only one usb port, and you want usb wakeup at this port, you
+can use below script to enable usb wakeup.
+for i in $(find /sys -name wakeup | grep usb);do echo enabled > $i;done;
+