usb: dwc3: of-simple: add compatible for rockchip rk3399

Rockchip platform merely enable usb3 clocks and
populate its children. So we can use this generic
glue layer to support Rockchip dwc3.

Signed-off-by: William Wu <william.wu@rock-chips.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>