usb: dwc2: move device tree bindings doc to correct place

Now that the DWC2 driver has been moved to drivers/usb, move its
bindings doc to the correct place

Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>