s390/irq: count cpu restart events

Count CPU Restart events and make them visible via /proc/interrupts.
Every CPU hotplug (online) event will increase the per cpu counter.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>