mtd: mxc_nand: fix OOB corruption when page size > 2KiB

When page size is 4KiB, ecc.total is set to 8*9, and this causes
nand_write_page_hwecc() to read past the initialized part of the eccpos array,
which corrupts chip->oob_poi with bogus values from ecc_calc.

Fix this by creating a proper nand_ecclayout for 4KiB flashes.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>