ASoC: rsnd: add SSIU BUSIF support for Document

Gen2 has BUSIF0-3, Gen3 has BUSIF0-7 on some SSIU.
Current driver is assuming it is using BUSIF0 as default.
Thus, SSI is attaching SSIU (with BUSIF0) by using rsnd_ssiu_attach().
But, TDM split mode also needs other BUSIF to use it.
This patch adds missing SSIU BUSIFx support.

BUSIF is handled by SSIU instead of SSI anymore.
Thus, its settings no longer needed on SSI node on DT.
This patch removes its settings from Document, but driver is still
keeping compatibility. Thus, old DT style is still working.
But, to avoid confusing, it doesn't indicate old compatibility things on
Document. New SoC should have SSIU on DT from this patch.

1) old style DT is still supported (= no rcar_sound,ssiu node on DT)
2) If ssiu is not indicated on playback/capture,
   BUSIF0 will be used as default
	playback = <&ssi3>; /* ssiu30 will be selected */
3) you can select own ssiu
	playback = <&ssi32 &ssi3>; /* ssiu32 will be selected */

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