s390/cio: add channel ID sysfs attribute

Add new attributes "chid" and "chid_external" to the channel-path
sysfs directory. These attributes contain information related to
the channel-ID of the channel-path.

Reviewed-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>