mtd: docg3: add write functions

Add write capability to the docg3 driver. The writes are
possible on a single page (512 bytes + 16 bytes), even if
that page is split on 2 physical pages on 2 blocks (each on
one plane).
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>