ASoC: rsnd: add sample code of dma entry

rsnd driver supports Audio DMAC (via DMAEngine) /
Audio DMAC peri peri (via rsnd driver) now.
This patch adds DT binding sample code on DT binging text

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