mtd: onenand: add bbt_wait & unlock_all as replaceable for some platform

Add bbt_wait & unlock_all as replaceable for some platform such as
  s3c64xx s3c64xx has its own OneNAND controller and another interface

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>