Documentation: dt-bindings: add dt binding info for dwc2 dr_mode

Indicate that the generic dr_mode binding should be used for dwc2.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>