mtd: nand/docg4: reserve bb marker area in ecclayout

Modify the nand_ecclayout to place the two bb marker bytes in the oob region
off-limits to the user.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>