mxc_nand: support 8bit ecc

Nand devices with at least 26 bytes of oob data per 512 byte block
can have 8bit ecc on v2 type controllers. This is currently not tested,
but at least this patch puts the ECC_MODE bit into a well defined state.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>