MLBEDSW-9865: MLCE: Add constraint on Softmax

 - Lowering of softmax can only be done as long as the product of
IFM width and height is within max allowed size for tensor dimension

Change-Id: Ifeacd3a8a8a9d7b1922a32b0475256d93a16fc8d
Signed-off-by: Johan Alfven <johan.alfven@arm.com>