ARM: at91: atmel_nand: Support flash based BBT

Add support for on-flash bad block table. This makes U-Boot handle an existing
BBT correctly.

Signed-off-by: David Dueck <davidcdueck@googlemail.com>
Reviewed-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
CC: Boris BREZILLON <boris.brezillon@free-electrons.com>
CC: Josh Wu <josh.wu@atmel.com>
CC: Andreas Bießmann <andreas.devel@googlemail.com>
CC: Scott Wood <scottwood@freescale.com>
Acked-by: Josh Wu <josh.wu@atmel.com>