mtd: nand_base: allow drivers to choose ECC block size

This patch allows core driver to choose ECC block size in
sw ecc case.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>