mtd: nand: Make flags for bad block marker position more granular

To be able to check and set bad block markers in the first and
second page of a block independently of each other, we create
separate flags for both cases.

Previously NAND_BBM_SECONDPAGE meant, that both, the first and the
second page were used. With this patch NAND_BBM_FIRSTPAGE stands for
using the first page and NAND_BBM_SECONDPAGE for using the second
page.

This patch is only for preparation of subsequent changes and does
not implement the logic to actually handle both flags separately.

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>