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>