ASoC: simple-audio-card: enable playback/capture_only property

Audio Graph Card2 is supporting playback/capture_only property,
let's follow Simple Audio Card, too.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://msgid.link/r/87wmnefrv4.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>