ARC: bump default NR_CPUS from 2 to 4

Currently we have up-to 4 cores in SMP so let's set default value
accordingly.

This change doesn't introduce any performance degradation for
dual-core HW while allows the same image to be executed on
quad-core HW.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>