MLBEDSW-2745 Support relus with differing scales

In the event we have a relu op with different input and output scales,
we need to fuse it with a nop avgpool.
Also refactor the existing avgpool nop code to a common function.

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: Iedf4513e7595ee4ee1777ba0b1eb38a8df8aed5e