mtd: nand: mxc: implement onfi get/set features

To be able to support different ONFI timing modes we have to implement
the onfi_set_features and onfi_get_features. Tested on an i.MX25 SoC.

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