dt-bindings: usb: dwc2: add description for px30

This patch adds the compatible of dwc2 for PX30 SoCs.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -6,6 +6,7 @@ Required properties:
   - brcm,bcm2835-usb: The DWC2 USB controller instance in the BCM2835 SoC.
   - hisilicon,hi6220-usb: The DWC2 USB controller instance in the hi6220 SoC.
   - rockchip,rk3066-usb: The DWC2 USB controller instance in the rk3066 Soc;
+  - "rockchip,px30-usb", "rockchip,rk3066-usb", "snps,dwc2": for px30 Soc;
   - "rockchip,rk3188-usb", "rockchip,rk3066-usb", "snps,dwc2": for rk3188 Soc;
   - "rockchip,rk3288-usb", "rockchip,rk3066-usb", "snps,dwc2": for rk3288 Soc;
   - "lantiq,arx100-usb": The DWC2 USB controller instance in Lantiq ARX SoCs;