ASoC: rsnd: tidyup for SSIU subnode

commit da48a6eb ("ASoC: rsnd: add SSIU BUSIF support for
Document") updated Documentation for SSIU, but 1) we want to
keep old/deprecated DMA description, 2) it is missing SSIU
subnode properties. This patch tidyup these

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