mtd: nand: sunxi: fix support for 512bytes ECC chunks

The driver is incorrectly assuming that the ECC block size is always 1k
which is not always true.

Also take the other cases into account.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>