mtd: nand: Provide nand_cleanup() function to free NAND related resources

Provide a nand_cleanup() function to free all nand related resources
without unregistering the mtd device.
This should allow drivers to call mtd_device_unregister() and handle
its return value and still being able to cleanup all nand related
resources.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Daniel Walter <dwalter@sigma-star.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>