ASoC: Add PDM DAI format definition

Add DAI format definition for PDM interfaces.
Signed-off-by: Misael Lopez Cruz <x0052729@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/sound/soc-dai.h

@@ -30,6 +30,7 @@ struct snd_pcm_substream;
 #define SND_SOC_DAIFMT_DSP_A		3 /* L data MSB after FRM LRC */
 #define SND_SOC_DAIFMT_DSP_B		4 /* L data MSB during FRM LRC */
 #define SND_SOC_DAIFMT_AC97		5 /* AC97 */
+#define SND_SOC_DAIFMT_PDM		6 /* Pulse density modulation */
 
 /* left and right justified also known as MSB and LSB respectively */
 #define SND_SOC_DAIFMT_MSB		SND_SOC_DAIFMT_LEFT_J