usb/doc: Add document for USB3 debug port usage

Add Documentation/usb/usb3-debug-port.rst. This document includes
the guide for using USB3 debug port.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-usb@vger.kernel.org
Link: http://lkml.kernel.org/r/1490083293-3792-6-git-send-email-baolu.lu@linux.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>