docs: add documentation for vfio-ccw

Add file Documentation/s390/vfio-ccw.txt that includes details
of vfio-ccw.
Acked-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Message-Id: <20170317031743.40128-15-bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

Documentation/s390/00-INDEX

@@ -22,5 +22,7 @@ qeth.txt
 	- HiperSockets Bridge Port Support.
 s390dbf.txt
 	- information on using the s390 debug feature.
+vfio-ccw.txt
+	  information on the vfio-ccw I/O subchannel driver.
 zfcpdump.txt
 	- information on the s390 SCSI dump tool.