mtd: rawnand: AMD: Also use the last page for bad block markers

According to the datasheet of some Cypress SLC NANDs, the bad
block markers can be in the first, second or last page of a block.
So let's check all three locations.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Boris Brezillon <bbrezillon@kernel.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>