Add support for VoltageMargin setting

Tested: Can set/get the voltage_margin setting
Change-Id: I8d1042a90ad75a312a30daae43e231edea195281
Signed-off-by: Gaurav Gandhi <gauravgandhi@google.com>
diff --git a/handler_impl.hpp b/handler_impl.hpp
index d671cc2..3d027f9 100644
--- a/handler_impl.hpp
+++ b/handler_impl.hpp
@@ -108,7 +108,10 @@
         {0x20, "memory_device"}};
 
     const std::unordered_map<uint8_t, std::string> _vrSettingsMap{
-        {0, "idle_mode_"}, {1, "power_brake_"}, {2, "loadline_"}};
+        {0, "idle_mode_"},
+        {1, "power_brake_"},
+        {2, "loadline_"},
+        {3, "vout_margin_"}};
 
     const std::unordered_map<uint8_t, std::string> bmInstanceTypeStringMap = {
         {0x00, "asset-tag"}, {0x01, "board-serial-number"},