atmel_nand: Add 32 bit ecc support for sama5d2 chip

Also if minimum ecc requirment is bigger then what we support, then just
use our maxium pmecc support.
But it is not safe, so we'll output a warning about this.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>