dt-bindings: soc: rockchip: grf: add compatible for RK3308 USB grf

The RK3308 has a USB GRF.
This patch adds a compatible string for it.
Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210601164800.7670-3-jbx6244@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

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

@@ -31,6 +31,7 @@ properties:
               - rockchip,rk3308-core-grf
               - rockchip,rk3308-detect-grf
               - rockchip,rk3308-grf
+              - rockchip,rk3308-usb2phy-grf
               - rockchip,rk3328-grf
               - rockchip,rk3328-usb2phy-grf
               - rockchip,rk3368-grf
@@ -174,6 +175,7 @@ allOf:
             enum:
               - rockchip,px30-usb2phy-grf
               - rockchip,rk3228-grf
+              - rockchip,rk3308-usb2phy-grf
               - rockchip,rk3328-usb2phy-grf
               - rockchip,rk3399-grf
               - rockchip,rv1108-grf