Change to TFLM main in external configuration

Use `main` branch from upstream to keep CMSIS-NN and TFLM
in sync

Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>
Change-Id: I609fd6229551fc8ef11f4821773cd02357bd9c9d
diff --git a/externals.json b/externals.json
index 27cb94f..3cfe0dc 100644
--- a/externals.json
+++ b/externals.json
@@ -58,7 +58,7 @@
         {
             "path": "core_software/tflite_micro",
             "fetchurl": "https://github.com/tensorflow/tflite-micro",
-            "revision": "80cb11b131e9738dc60b2db3e2f1f8e2425ded52"
+            "revision": "main"
         },
         {
             "path": "linux_driver_stack",