ASoC: rsnd: ssi: add rsnd_ssi_of_node()

This patch adds rsnd_ssi_of_node() to get SSI subnode from DT.
This is prepare for new DT bindings for 1st DMAC

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