documentation: update s390 header file paths

Update Documentation/s390/ files to reflect changed header files
locations.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>