s390/cio: export CHPID operating speed

Add a per-CHPID sysfs attribute named "speed_bps" that provides the
operating speed of the associated channel path in bits per second,
or 0 if the operating speed is not available.

Example:

$ cat /sys/devices/css0/chp0.32/speed_bps
32G

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