s390/cio: move struct node_descriptor to cio.h

This allows device drivers (eg. qeth) to use the struct when processing
information retrieved via RCD.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>