dt-bindings: document PX30 usb2phy General Register Files

One of the separate General Register Files contains the registers for
controlling the usb2phy, so add the necessary binding compatible for it.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190917082659.25549-11-heiko@sntech.de

Documentation/devicetree/bindings/soc/rockchip/grf.txt

@@ -30,6 +30,7 @@ Required Properties:
 - compatible: SGRF should be one of the following
    - "rockchip,rk3288-sgrf", "syscon": for rk3288
 - compatible: USB2PHYGRF should be one of the followings
+   - "rockchip,px30-usb2phy-grf", "syscon": for px30
    - "rockchip,rk3328-usb2phy-grf", "syscon": for rk3328
 - compatible: USBGRF should be one of the following
    - "rockchip,rv1108-usbgrf", "syscon": for rv1108