ASoC: rsnd: add null CLOCKIN support

Some Renesas SoC doesn't have full CLOCKIN.
This patch add null_clk, and accepts it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87tumsoe2p.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>