ASoC: rsnd: add missing reg/reg-name at Document

reg is missing "Audio DMAC peri peri" part,
and reg-name itself is missing at Document.
This patch add these.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@kernel.org>