Documentation: ABI: usb: sysfs Description for chipidea USB OTG HNP and SRP

This patch adds sysfs interface description for chipidea USB OTG HNP and SRP.

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>