[MTD] [NAND] nand_ecc.c: adding support for 512 byte ecc

Support 512 byte ECC calculation

[FM: updated two comments]

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>