ASoC: dt-bindings: audio-graph-port: Document new DAI link flags playback-only/capture-only

Document new playback-only and capture-only flags which can be used
when dai link can only support just one direction: playback or capture
but not both.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://msgid.link/r/20231128081119.106360-3-daniel.baluta@oss.nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>