s390: sclp event support

s390: sclp event support

Several SCLP features are considered to be events. Those events don't
provide SCLP commands on their own, instead they are all based on
Read Event Data, Write Event Data, Write Event Mask and the service
interrupt. Follow-on patches will provide SCLP's Signal Quiesce (via
system_powerdown) and the ASCII console.
Further down the road the sclp line mode console and configuration
change events (e.g. cpu hotplug) can be implemented.

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>