ASoC: audio-graph-card2-custom-sample.dtsi: add verbose explanation

audio-graph-card2-custom-sample.dtsi will be used to test
Audio-Graph-Card2 behavior. But it is difficult to say that it is easy
to understand, because the comment/explanation are not so many.

This patch add verbose explanation to it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87czeps95h.wl-kuninori.morimoto.gx@renesas.com


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