ASoC: soc-pcm: add soc_create_pcm() and simplify soc_new_pcm()

soc_new_pcm() implementation is very long / verbose / complex,
thus, it is very difficult to read.
If we read it carefully, we can notice that it is consisted by

	int soc_new_pcm(...)
	{
		(1) judging playback/caputre part
		(2) creating the PCM part
		(3) setup pcm/rtd part
	}

This patch adds new soc_create_pcm() for (2) part
and offload it from snd_pcm_new().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/875z3paigi.wl-kuninori.morimoto.gx@renesas.com


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