s390/Docs: new doc describing lock usage by the vfio_ap device driver

Introduces a new document describing the locks used by the vfio_ap device
driver and how to use them so as to avoid lockdep reports and deadlock
situations.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>