ASoC: rockchip-spdif: add #sound-dai-cells property

'#sound-dai-cells' is required to properly interpret
the list of DAI specified in the 'sound-dai' property,
so add them to 'rockchip-spdif.yaml'
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200404115225.4314-2-jbx6244@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/rockchip-spdif.yaml

@@ -57,6 +57,9 @@ properties:
       The phandle of the syscon node for the GRF register.
       Required property on RK3288.
 
+  "#sound-dai-cells":
+    const: 0
+
 required:
   - compatible
   - reg
@@ -65,6 +68,7 @@ required:
   - clock-names
   - dmas
   - dma-names
+  - "#sound-dai-cells"
 
 if:
   properties:
@@ -90,4 +94,5 @@ examples:
       clock-names = "mclk", "hclk";
       dmas = <&dmac1_s 8>;
       dma-names = "tx";
+      #sound-dai-cells = <0>;
     };