kconfig: Remove MP_NUM_CPUS usage

Zephyr's code base uses MP_MAX_NUM_CPUS to
know how many cores exists in the target. It is
also expected that both symbols MP_MAX_NUM_CPUS
and MP_NUM_CPUS have the same value, so lets
just use MP_MAX_NUM_CPUS and simplify it.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>