s390/zcrypt: fix suspend/resume of AP bus devices

If there are no devices on the AP bus there will not be a single
call to the per-device ap_bus_suspend function. Even worse,
there will not be a call to the per-device ap_bus_resume either
and the AP will fail so resume correctly.

Introduce a bus specific dev_pm_ops to suspend / resume the AP
bus related things. While we are at it, simplify the power management
code of the AP bus.

Reviewd-by: Ingo Tuchscherer <ingo.tuchscherer@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>