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: 889183
Change-Id: I6685dd0f346072029ca51e4ce92856a13e74ee76
diff --git a/hal/msm8916/platform.h b/hal/msm8916/platform.h
index bf5e834..5d0ad2d 100644
--- a/hal/msm8916/platform.h
+++ b/hal/msm8916/platform.h
@@ -236,6 +236,7 @@
 #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