hal: add support for ull audio playback

Add a new use case and mixer paths to support ultra
low latency audio playback. This use case exercises the
ULL mode in the DSP.

CRs-Fixed: 892056
Change-Id: I6685dd0f346072029ca51e4ce92856a13e74ee76
diff --git a/hal/msm8974/platform.h b/hal/msm8974/platform.h
index b8f8cee..d18bd7c 100644
--- a/hal/msm8974/platform.h
+++ b/hal/msm8974/platform.h
@@ -217,6 +217,7 @@
 #define DEEP_BUFFER_PCM_DEVICE 0
 #define AUDIO_RECORD_PCM_DEVICE 0
 #define MULTIMEDIA2_PCM_DEVICE 1
+#define MULTIMEDIA3_PCM_DEVICE 4
 #define FM_PLAYBACK_PCM_DEVICE 5
 #define FM_CAPTURE_PCM_DEVICE  6
 #define HFP_PCM_RX 5