ASoC: rockchip: add bindings for rk3368 i2s

Add devicetree bindings for i2s controller found on rk3368
processors from rockchip.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -9,6 +9,7 @@ Required properties:
    - "rockchip,rk3066-i2s": for rk3066
    - "rockchip,rk3188-i2s", "rockchip,rk3066-i2s": for rk3188
    - "rockchip,rk3288-i2s", "rockchip,rk3066-i2s": for rk3288
+   - "rockchip,rk3368-i2s", "rockchip,rk3066-i2s": for rk3368
    - "rockchip,rk3399-i2s", "rockchip,rk3066-i2s": for rk3399
 - reg: physical base address of the controller and length of memory mapped
   region.