MLBEDSW-3771: Updated to TF 2.4 flatbuffer schema

Updated tflite loader and mappings from tensorflow 2.3 to tensorflow 2.4

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I55884000ee139baf639bb0377008e0534f72fe94
diff --git a/ethosu/vela/tflite/BuiltinOptions.py b/ethosu/vela/tflite/BuiltinOptions.py
index 6f0aab0..6be2d43 100644
--- a/ethosu/vela/tflite/BuiltinOptions.py
+++ b/ethosu/vela/tflite/BuiltinOptions.py
@@ -105,4 +105,5 @@
     DensifyOptions = 99
     SegmentSumOptions = 100
     BatchMatMulOptions = 101
+    CumsumOptions = 102