ASoC: ak4642: enable stereo line output power-save mode

ak4642 has power-save mode for stereo line to reduce pop noise.
This patch enables it.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>