ASoC: rsnd: add simplified module explanation

Renesas sound driver user needs to read its datasheet when create DT.
But it is difficult to understand, because it has many modules
(SRC/CTU/MIX/DVC/SSIU/SSI/AudioDMAC/AudioDMACperiperi),
and many features (Asynchronous/Synchronous mode on SRC, CTU matrix,
DVC volume settings feature, Multi-SSI/TDM-SSI, etc).
This patch adds simplified explanation to help setting/understanding.

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