Update mbed to 5.9.4

Author: ivankravets

platform.json

@@ -31,7 +31,7 @@
     "framework-mbed": {
       "type": "framework",
       "optional": true,
-      "version": "~4.50903.0"
+      "version": "~5.50904.0"
     },
     "tool-openocd": {
       "type": "debugger",