ASoC: soc-pcm: Allow to specify unidirectional dai_link

Add 'playback_only' and 'capture_only' fields that can be used for specifying
that a dai_link has a unidirectional capability.

The motivation for this is for the cases of systems, such as Freescale MX28,
that has two unidirectional DAIs.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
diff --git a/sound/soc/soc-pcm.c b/sound/soc/soc-pcm.c
index b6c6403..9abaa52 100644
--- a/sound/soc/soc-pcm.c
+++ b/sound/soc/soc-pcm.c
@@ -2027,6 +2027,16 @@
 			capture = 1;
 	}
 
+	if (rtd->dai_link->playback_only) {
+		playback = 1;
+		capture = 0;
+	}
+
+	if (rtd->dai_link->capture_only) {
+		playback = 0;
+		capture = 1;
+	}
+
 	/* create the PCM */
 	if (rtd->dai_link->no_pcm) {
 		snprintf(new_name, sizeof(new_name), "(%s)",