ASoC: rsnd: Get correct SCU ID

Current rsnd driver is assuming that SCU/SRU ID is
same as SSIU/SSI ID, because Gen1 can't select it.
But, Gen2 can select it.
The SCU/SRU/SSIU/SSI pair depends on the platform.
This patch get correct SCU ID from platform info.
To keep compatible, it still assuming SCU ID = SSI ID
if platform doesn't have info

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