s390/smp: Switch to GENERIC_CPU_DEVICES

Instead of setting up non-boot CPUs early in architecture code,
only setup the cpu present mask and let the generic code handle
cpu bringup.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>