COMPMID-3442: Add support of negative axis in NESoftmaxLayer and reference code

COMPMID-3442: Add support of negative axis in NESoftmaxLayer and reference code

Signed-off-by: Sheri Zhang <sheri.zhang@arm.com>
Change-Id: I285cc3b74ac0a45f0ad5830baed5237cea568f15
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3147
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>