[S390] ap bus: add support for irq statistics

Add support for AP Bus I/O interrupt statistics in /proc/interrupts.

Signed-off-by: Holger Dengler <hd@linux.vnet.ibm.com>
Signed-off-by: Felix Beck <felix.beck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>