mtd: rawnand: Rename nand_default_bbt() into nand_create_bbt()

Rename nand_default_bbt() into nand_create_bbt() and pass it a nand_chip
object to prepare removal of the chip->scan_bbt() hook.

We add a temporary nand_default_bbt() wrapper which will be dropped
after the removal of ->scan_bbt().

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>