MLBEDSW-3694 Replace padding with enum

Use an Enum instead of a bytestring to specify VALID or SAME padding

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: I4e87f8c32b3bfac176d822a68de061e85a558fce
diff --git a/ethosu/vela/tflite_mapping.py b/ethosu/vela/tflite_mapping.py
index fe58261..cc6053c 100644
--- a/ethosu/vela/tflite_mapping.py
+++ b/ethosu/vela/tflite_mapping.py
@@ -24,6 +24,7 @@
 from .data_type import DataType
 from .operation import CustomType
 from .operation import Op
+from .operation import Padding as opPad
 from .tflite import AbsOptions
 from .tflite import AddNOptions
 from .tflite import AddOptions
@@ -425,8 +426,8 @@
 
 
 padding_map = {
-    Padding.SAME: b"SAME",
-    Padding.VALID: b"VALID",
+    Padding.SAME: opPad.SAME,
+    Padding.VALID: opPad.VALID,
 }
 
 padding_inv_map = inverse_map(padding_map)