boards: ARC: nsim: simplify runner setup & pass MP_NUM_CPUS unconditionally

Do not check for exact SoC to pass '--cores=${CONFIG_MP_NUM_CPUS}'
options to runners an pass it unconditionally instead.

That preventis from issues when adding new SoC/board.

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