ARC: west: mdb runner: support 12 cores targets

Bump number of cores limit in mdb west runner.
As we are here - adjust core number in runner test closer to
the limit.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>