[S390] register cpu devices for SMP=n

A kernel compiled with SMP=n will not register any cpu devices.
The resulting kernel image will not boot with this error message:

    kernel BUG at fs/sysfs/group.c:65!

Use GENERIC_CPU_DEVICES=y if SMP=n to get the missing cpu device.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>