ATMEL: fix nand ecc support

So we can now choose for the board the ecc mode (ecc soft, soft bch, no ecc
and hardware).

Set ecc mode in the boards to soft as currently in the driver.

Move platform data to a common header
include/linux/platform_data/atmel_nand.h

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Acked-by: David Woodhouse <dwmw2@infradead.org>