MLBEDSW-1970: Add stride 3 support

This patch adds support for strides of size 3.

It removes some obsolete code for a corner case that
no longer exists.
It also changes the setting of the bitfield in
NPU_SET_KERNEL_STRIDE so that it matches the specification.

Change-Id: I7dabcf72b7826ca0b3c98e9d23209027204079a8
Signed-off-by: Dwight Lidman <dwight.lidman@arm.com>
diff --git a/ethosu/vela/supported_operators.py b/ethosu/vela/supported_operators.py
index fb3061f..1e11d78 100644
--- a/ethosu/vela/supported_operators.py
+++ b/ethosu/vela/supported_operators.py
@@ -142,7 +142,7 @@
 
     def check_convolution_restrictions(self, op):
         # check stride
-        if op.attrs["stride_w"] > 2 or op.attrs["stride_h"] > 2:
+        if op.attrs["stride_w"] > 3 or op.attrs["stride_h"] > 3:
             return False
 
         # check dilation
@@ -182,7 +182,7 @@
 
     def check_pooling_restrictions(self, op):
         # check stride
-        if op.attrs["stride_w"] > 2 or op.attrs["stride_h"] > 2:
+        if op.attrs["stride_w"] > 3 or op.attrs["stride_h"] > 3:
             return False
 
         # check data type