Block config optimisation for 256/512 configurations

 - 256 and 512 configuration variants execute 1D convolutions
   in an optimised manner compared to their 2x2 microblock
   dimensions. This commit takes this into account to improve
   Conv1D throughput on these configurations.

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I6ecdf6e4a219e356327b22f8393f50ee8817af23