ASoC: rockchip-i2s: add description for px30

Add "rockchip,px30-i2s", "rockchip,rk3066-i2s" for i2s on px30 platform.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liang Chen <cl@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/rockchip-i2s.txt

@@ -7,6 +7,7 @@ Required properties:
 
 - compatible: should be one of the following:
    - "rockchip,rk3066-i2s": for rk3066
+   - "rockchip,px30-i2s", "rockchip,rk3066-i2s": for px30
    - "rockchip,rk3036-i2s", "rockchip,rk3066-i2s": for rk3036
    - "rockchip,rk3188-i2s", "rockchip,rk3066-i2s": for rk3188
    - "rockchip,rk3228-i2s", "rockchip,rk3066-i2s": for rk3228