mxc nand: use buffers

The NAND controller has some limitations how to access the
internal buffers. It only allows 32 bit accesses. The driver
used to work around this by having special alignment aware
copy routines.
We now copy the whole page to a buffer in memory and let the
access functions use this buffer. This simplifies the driver.
A bonnie++ test showed that this has no negative performance
impact on the driver.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>