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

This add necessary dwc2 binding documentation for Rockchip socs:
rk3066, rk3188 and rk3288
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -4,6 +4,9 @@ Platform DesignWare HS OTG USB 2.0 controller
 Required properties:
 - compatible : One of:
   - brcm,bcm2835-usb: The DWC2 USB controller instance in the BCM2835 SoC.
+  - rockchip,rk3066-usb: The DWC2 USB controller instance in the rk3066 Soc;
+  - "rockchip,rk3188-usb", "rockchip,rk3066-usb", "snps,dwc2": for rk3188 Soc;
+  - "rockchip,rk3288-usb", "rockchip,rk3066-usb", "snps,dwc2": for rk3288 Soc;
   - snps,dwc2: A generic DWC2 USB controller with default parameters.
 - reg : Should contain 1 register range (address and length)
 - interrupts : Should contain 1 interrupt