ASoC: ak4613: add single-end optional property for IN/OUT pins

ak4613 IN/OUT pin can be selected as differential/single-end.
Default is differential, because it is register default settings.

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