usb: dwc3: Add debugfs entry to toggle compliance pattern

USB 3.x transmitter electrical compliance testing requires measurements of
different compliance patterns that the PHY is able to output. Changing to
different patterns normally requires triggering an external signal to the
receiver to cause the controller to advance to the next pattern. It is
often useful to be able to toggle the patterns from the DUT itself. This
patch adds a debugfs entry to invoke the dwc3 controller to change
patterns when it is in compliance mode.

Change-Id: If5f7ac793b0c3f0c8ab1775cf7cbe7b48176c325
Signed-off-by: Ronak Vijay Raheja <rraheja@codeaurora.org>