ASoC: rsnd: Document R-Car M3-W support

Document support for the sound modules in the Renesas M3-W (r8a7796)
SoC.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>