mtd: mxc_nand: use a flag to detect if the mx21 quirk is necessary

This gets rid of several instances of cpu_is_mx21() in the driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>