MLBEDSW-5284 - MLCE: MEAN batch size constraint

Adds a constraint on batch size for the MEAN
operator. Due to the restrictions imposed by the
NHCWB16 format, some operators do not allow batch
sizes larger than 1. The MEAN operator uses the
operators in its implementation, but the
constraint was missing and thus the operator was
being wrongly placed on the NPU.

Signed-off-by: Dwight Lidman <dwight.lidman@arm.com>
Change-Id: I95e07a9151a6a131c337caa3114da1154be39e49