hal: add support for external FM volume control

APQ8084 uses an external chip for FM. Add support for
mixer control for external FM volume control.

Change-Id: I296d0b32fce632a296057e6d7b48698d9b0648fe
diff --git a/hal/msm8916/platform.h b/hal/msm8916/platform.h
index f081912..f12697c 100644
--- a/hal/msm8916/platform.h
+++ b/hal/msm8916/platform.h
@@ -178,6 +178,8 @@
 #define PLAYBACK_OFFLOAD_DEVICE 9
 #define COMPRESS_VOIP_CALL_PCM_DEVICE 3
 
+/* Define macro for Internal FM volume mixer */
+#define FM_RX_VOLUME "Internal FM RX Volume"
 
 #define LOWLATENCY_PCM_DEVICE 12
 #define EC_REF_RX "I2S_RX"