usb: chipidea: msm: Add device tree binding information

Document device tree binding information as required by
the Qualcomm USB controller.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>