mtd: docg3: add OOB buffer to device structure

Add OOB buffer area to store the OOB data until the actual
page is written, so that it can be completed by hardware ECC
generator.

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>