Fix RAM size for M5Stack Core2 and M5Stack FIRE

Resolve platformio#596

Author: valeros

boards/m5stack-core2.json

@@ -25,7 +25,7 @@
   "name": "M5Stack Core2",
   "upload": {
     "flash_size": "16MB",
-    "maximum_ram_size": 6553600,
+    "maximum_ram_size": 4521984,
     "maximum_size": 16777216,
     "require_upload_port": true,
     "speed": 460800

boards/m5stack-fire.json

@@ -25,7 +25,7 @@
   "name": "M5Stack FIRE",
   "upload": {
     "flash_size": "16MB",
-    "maximum_ram_size": 6553600,
+    "maximum_ram_size": 4521984,
     "maximum_size": 16777216,
     "require_upload_port": true,
     "speed": 460800