ASoC: rsnd: Document optional reset properties

Document the optional properties for describing module resets, to
support resetting these modules on R-Car Gen2 and Gen3.

Note that the audio module has resets for the Serial Sound Interfaces
only.

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