nios2: Add missing NR_CPUS to Kconfig

kernel/time/Kconfig expects that NR_CPUS is defined.

This fixes the following config warning:
"kernel/time/Kconfig:163:warning: range is invalid"

Follow 4cbbbb43 ("microblaze: Fix missing NR_CPUS in menuconfig")

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>