ASoC: audio-graph-card2: add comment for format property

We don't need to have "format" property on DT any more if
CPU/Codec driver has .auto_selectable_formats settings
on snd_soc_dai_ops. The sample dtsi doesn't have it.
To avoid user confusion, this patch indicates it on comment.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87edjuzk2p.wl-kuninori.morimoto.gx@renesas.com


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