s390/smp: disallow CPU hotplug of CPU 0

On s390, CPU 0 has special properties in comparison to other CPUs, as it
cannot be deconfigured for example. Therefore, allowing to hotplug CPU 0
introduces additional complexity when handling these properties.
Disallowing to hotplug CPU 0 allows to remove such complexities.

This follows x86 which also prevents offlining of CPU0 since commit
e59e74dc ("x86/topology: Remove CPU0 hotplug option").

[hca@linux.ibm.com: changed commit message]
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>