ASoC: core: Add support for machine specific trigger callback

Machine specific trigger callback allows to do final stream start/stop
related operations in a machine driver after setting up the codec, DMA and
DAI.

One example could be clock management for linked streams case where machine
driver can start/stop synchronously the linked streams.
Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/soc-pcm.c

@@ -819,6 +819,13 @@ static int soc_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
 		if (ret < 0)
 			return ret;
 	}
+
+	if (rtd->dai_link->ops && rtd->dai_link->ops->trigger) {
+		ret = rtd->dai_link->ops->trigger(substream, cmd);
+		if (ret < 0)
+			return ret;
+	}
+
 	return 0;
 }