Sometimes, we want to control which of the matching drivers
binds to a subchannel device (e.g. for subchannels we want to
handle via vfio-ccw).

For pci devices, a mechanism to do so has been introduced in
782a985d ("PCI: Introduce new device binding path using
pci_dev.driver_override"). It makes sense to introduce the
driver_override attribute for subchannel devices as well, so
that we can easily extend the 'driverctl' tool (which makes
use of the driver_override attribute for pci).

Note that unlike pci we still require a driver override to
match the subchannel type; matching more than one subchannel
type is probably not useful anyway.
Signed-off-by: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NHalil Pasic <pasic@linux.ibm.com>
Reviewed-by: NSebastian Ott <sebott@linux.ibm.com>
Signed-off-by: NSebastian Ott <sebott@linux.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>