mxc_nand: make some internally used functions overwriteable

This patch prepares the driver to add v3 controller support
later. The v3 controller is basically the same controller as v1
and v2, but with a completely different register layout.

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