ASoC: rockchip-spdif: add power-domains property

In the old txt situation we add/describe only properties that are used
by the driver/hardware itself. With yaml it also filters things in a
node that are used by other drivers like 'power-domains' for rk3399,
so add it 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/20200404115225.4314-3-jbx6244@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -51,6 +51,9 @@ properties:
   dma-names:
     const: tx
 
+  power-domains:
+    maxItems: 1
+
   rockchip,grf:
     $ref: /schemas/types.yaml#/definitions/phandle
     description: