mtd: s3c2410: make ecc mode configurable via platform data

Removing CONFIG_MTD_NAND_S3C2410_HWECC option and adding a ecc_mode
field in the drivers's platform data structure so it can be selectable
via platform data.

Also setting this field to NAND_ECC_SOFT in all boards using this
driver since none of them had CONFIG_MTD_NAND_S3C2410_HWECC enabled.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>