s390/cio: introduce CIO debugfs directory

This patch introduces an s390 Common I/O layer debugfs directory that is
intended to provide access to debugging-related CIO functions and data.
The directory is created as /sys/kernel/debug/s390/cio

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>