Documentation: usb: add chipidea.txt for how to demo usb OTG HNP and SRP

This patch adds a file chipidea.txt for how to demo chipidea usb OTG HNP and SRP
functions via sysfs input files, any other possible information should be
documented for chipidea usb driver in future can be added into this file.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Li Jun <b47624@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>