s390x/css: introduce read-only property type for device ids

Let's introduce a read-only property type that handles device ids of the
CssDevId type used for channel devices for future use. e.g. exposing the
busid of an I/O subchannel that is assigned to a ccw device.

Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
