ASoC: soc-utils: add snd_soc_dlc_is_dummy()

Merge series from Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>:

We are using dummy component/dlc, but didn't have its check funciton.
This patch adds it.

diff --cc include/sound/soc.h
Simple merge

diff --cc sound/soc/mediatek/mt8195/mt8195-mt6359.c
Simple merge