ASoC: rsnd: rsnd_reg cleanup for SSIU

R-Car Gen1 didn't have SSIU IP, and it was part of SRU.
In Gen2, SSIU was created and it has original register.
Let's cleanup rsnd_reg for SSIU, because this driver
doesn't support Gen1 SRU any more.

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