ASoC: core: add snd_soc_of_parse_audio_prefix()

Current ASoC can add name_prefix for DAPM, and it is necessary for
route settings. This patch adds snd_soc_of_parse_audio_prefix() for
this purpose. It will be used with snd_soc_of_parse_audio_routing().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>