s390/ap: Support driver_override for AP queue devices

s390/ap: Support driver_override for AP queue devices

Add a new sysfs attribute driver_override the AP queue's
directory. Writing in a string overrides the default driver
determination and the drivers are matched against this string
instead. This overrules the driver binding determined by the
apmask/aqmask bitmask fields.

According to the common understanding of how the driver_override
behavior shall work, there is no further checking done. Neither about
the string which is given as override driver nor if this device is
currently in use by an mdev device. Another patch may limit this
behavior to refuse a mixed usage of the driver_override and
apmask/aqmask feature.

As there exists some tooling for this kind of driver_override
(see package driverctl) the AP bus behavior for re-binding
should be compatible to this. The steps for a driver_override are:
 1) unbind the current driver from the device. For example
    echo "17.0005" > /sys/devices/ap/card17/17.0005/driver/unbind
 2) set the new driver for this device in the sysfs
    driver_override attribute. For example
    echo "vfio_ap" > /sys//devices/ap/card17/17.0005/driver_override
 3) trigger a bus reprobe of this device. For example
    echo "17.0005" > /sys/bus/ap/drivers_probe
With the driverctl package this is more comfortable and
the settings get persisted:
  driverctl -b ap set-override 17.0005 vfio_ap
and unset with
  driverctl -b ap unset-override 17.0005

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>