ARC: boards: nsim_hs: add missing MPU enabling

We mark MPU existing for nSIM HS SoC, but doesn't enable
it for nsim_hs board. Fix that.

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