ASoC: rsnd: add rsnd_set_slot() / rsnd_get_slot_num()

TDM will use 6 or 8 slots on 1 SSI, and Multi channel will use
6 or 8 slots on few SSI (each SSI uses 2 slots).
Thus, this adds new slot control functions which can be prepare
for Multi channel support.

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