MLBEDSW-5554: Constraints for single-axis mean operations on NPU

 - Combine two MEAN operator checks for single axis averages into one
 - Only apply that check if the single axis is the height dimension
   (previously checks were also applied to width averages)
 - Rephrase some MEAN operator constraint descriptions

Signed-off-by: James Peet <james.peet@arm.com>
Change-Id: Ie0577f2b99aba1f3d6a4c39f8934eafe3813b736