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.com


Signed-off-by: Mark Brown <broonie@kernel.org>