add snd_soc_{of_}get_dlc()

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

Current soc-core.c has snd_soc_{of_}get_dai_name() to get DAI name
for dlc (snd_soc_dai_link_component). It gets .dai_name, but we need
.of_node too. Therefor user need to arrange.

It will be more useful if it gets both .dai_name and .of_node.
This patch adds snd_soc_{of_}get_dlc() for it, and convert to use it.