s390/ap: let bus_register() add the AP bus sysfs attributes

Instead of creating the sysfs attributes for the AP bus by hand,
describe them in .bus_groups and let the driver core handle it.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-my: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>