s390: move memory calculation into the sclp device

The restrictions for memory calculation belong to the sclp device.

Let's move the calculation to that point, so we are able to unify it for
both s390 machines. The sclp device is the first device to be initialized.
It performs the calculation and safely stores it in the machine, where
other parts of the system can access an reuse it.

The memory hotplug device is now only created when it is really needed.

Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
