AT91SAM9260EK: Handle 8 or 16 bit NAND

The Atmel boards can handle 8 or 16 bit NAND memories. This patch
makes the support configurable in the board config header file
(CFG_NAND_DBW_8 or CFG_NAND_DBW_16).

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>