mtd: introduce function max_bad_blocks

If implemented, 'max_bad_blocks' returns the maximum number of bad
blocks to reserve for a MTD. An implementation for NAND is coming soon.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Signed-off-by: Zach Brown <zach.brown@ni.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electron.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>