mtd: rawnand: Deprecate ->block_{bad,markbad}() hooks

Those hooks have been overloaded by some drivers for bad reasons:
either the driver was not fitting in the NAND framework and should
have been an MTD driver (docg4), or it was not properly implementing
the OOB read/write request or had a weird layout where BBM are trashed.
In any case, we should discourage people from overloading those
methods and encourage them to fix their driver instead.

Move the ->block_{bad,markbad}() hooks to the nand_legacy struct to
make it clear.

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