Update default upload speed for m5stack-atom board (platformio#509)

Author: kasraghu

boards/m5stack-atom.json

@@ -27,7 +27,7 @@
     "maximum_ram_size": 327680,
     "maximum_size": 4194304,
     "require_upload_port": true,
-    "speed": 460800
+    "speed": 1500000
   },
   "url": "http://www.m5stack.com",
   "vendor": "M5Stack"