at91: remove MTD_NAND_ATMEL_BUSWIDTH_16 option

no board configure it as 'n' and it's an issue to merge all defconfigs
in one

On AT91SAM926x boards both types of NAND flash can be present
(8 and 16 bit data bus width).

so will pass it via system_rev

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Patrice Vilchez <patrice.vilchez@atmel.com>