ARC: SMP: fix IDU mask setup

Fix IDU mask setup:
 * fix GENMASK usage to avoid generating mask to one extra cpu (which
   doesn't exist in configuration)
 * use arch_num_cpus() instead of CONFIG_MP_NUM_CPUS to allow having
   some cpu's disabled (with detection in runtime)

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
(cherry picked from commit 16e9362b)