s390/pci: Expose firmware provided UID Checking state in sysfs

The sysfs file /sys/bus/pci/devices/<device_id>/uid_is_unique provides
the UID Checking state as a per device attribute, highlighting its
effect of providing the guarantee that a device's UID is unique.
As a device attribute, this parameter is however unavailable if no
device is configured.

Expose the UID Checking state as:
  - A platform-level parameter
  - Available regardless of device presence or state

Signed-off-by: Ramesh Errabolu <ramesh@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>