s390/net: add pnetid support

s390 hardware supports the definition of a so-call Physical NETwork
IDentifier (short PNETID) per network device port. These PNETIDS
can be used to identify network devices that are attached to the same
physical network (broadcast domain).

This patch provides the interface to extract the PNETID of a port of
a device attached to the ccw-bus or pci-bus.

Parts of this patch are based on an initial implementation by
Thomas Richter.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>