s390: remove s390_root_dev_*()

Replace s390_root_dev_register() with root_device_register() etc.

[Includes fix from Cornelia Huck]

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>