mtd: docg3: add OOB layout to mtdinfo

Add OOB layout description for docg3, so that userspace can
use this information to setup the data for write_oob().

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-by: Ivan Djelic <ivan.djelic@parrot.com>
Reviewed-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>