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/msm8974/platform.h b/hal/msm8974/platform.h
index 3bd218c..209ee2b 100644
--- a/hal/msm8974/platform.h
+++ b/hal/msm8974/platform.h
@@ -245,6 +245,12 @@
 #define HFP_ASM_RX_TX 24
 #endif
 
+#ifdef PLATFORM_APQ8084
+#define FM_RX_VOLUME "Quat MI2S FM RX Volume"
+#else
+#define FM_RX_VOLUME "Internal FM RX Volume"
+#endif
+
 #define LIB_CSD_CLIENT "libcsd-client.so"
 /* CSD-CLIENT related functions */
 typedef int (*init_t)(bool);