[EFM32] Set default baud rate to 115200.

Author: asmellby

mbed_lib.json

@@ -10,5 +10,10 @@
             "help": "Baud rate for stdio",
             "value": 9600
         }
+    },
+    "target_overrides": {
+        "EFM32": {
+            "stdio-baud-rate": 115200
+        }
     }
 }