ASoC: add and use asoc_dummy_dlc

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

Many ASoC drivers are using dummy DAI.
I have 2 concern about it. 1st one is there is no guarantee that local
strings ("snd-soc-dummy-dai",  "snd-soc-dummy") are kept until the card
was binded if it was added at subfunction.
2nd one is we can use common snd_soc_dai_link_component for it.
This patch-set adds common asoc_dummy_dlc, and use it.