s390/pci: Expose new port attribute for PCIe functions

Add SysFS attribute that provides the port number for PCI functions
representing a single port of a multi-port device.

Signed-off-by: Alexander Schmidt <alexs@linux.ibm.com>
Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>