MLBEDSW-6312: Find block config improvement

- The number of accumulators is doubled in an Ethos-U configuration with
  2 cores
- Likewise, for elementwise, depthwise and pooling operations
  the IFM buffer depth capacity is doubled
- FindBlock: step the search space depth in multiples of ublock * ncores

Change-Id: I923cc347a2f252876d405ed93095d39181103f81
Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>