ASoC: dt-bindings: fsl,micfil: Document #sound-dai-cells

imx8mp.dtsi passes #sound-dai-cells = <0> in the micfil node.

Document #sound-dai-cells to fix the following schema warning:

audio-controller@30ca0000: '#sound-dai-cells' does not match any of the regexes: 'pinctrl-[0-9]+'
from schema $id: http://devicetree.org/schemas/sound/fsl,micfil.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20231004122935.2250889-1-festevam@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl,micfil.yaml

@@ -56,6 +56,9 @@ properties:
       - const: clkext3
     minItems: 2
 
+  "#sound-dai-cells":
+    const: 0
+
 required:
   - compatible
   - reg