ASoC: dt-bindings: renesas,rsnd: #sound-dai-cells is not mandatory

Current renesas,rsnd is requesting #sound-dai-cells, but it is
needed in case of it is using "simple-card", but not needed in case of
"audio-graph". We will get below warning without this patch.
This patch fiup it.

${LINUX}/arch/arm64/boot/dts/renesas/r8a77950-salvator-x.dtb: sound@ec500000: '#sound-dai-cells' is a required property
	From schema: ${LINUX}/Documentation/devicetree/bindings/sound/renesas,rsnd.yaml
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87edrl7rf4.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/renesas,rsnd.yaml

@@ -64,6 +64,7 @@ properties:
     description: |
       it must be 0 if your system is using single DAI
       it must be 1 if your system is using multi  DAIs
+      This is used on simple-audio-card
     enum: [0, 1]
 
   "#clock-cells":
@@ -282,7 +283,6 @@ required:
   - reg-names
   - clocks
   - clock-names
-  - "#sound-dai-cells"
 
 allOf:
   - $ref: dai-common.yaml#