Doc: use misc-devices/ dir for drivers

We have a directory for misc drivers documentation, let's use it.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>
Cc: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>