ASoC: rsnd: add detail explanation of L/R conversion timing

Renesas Sound device *Hardware* L/R and Linux *Software* L/R are
inverted. Because of this background, it needs to convert L/R.
Then, DVC needs *Hardware* L/R, and Linux needs *Software* L/R.
Because Playback/Capture needs different timing, and there is no
explanation about it on source code / git log, this patch adds it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>