mtd: nand: mxc: Add function to control hardware ECC

For proper raw read/write support need to be able to control the
hardware ECC engine. Add a function to enable/disable it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>