mtd: rawnand: mxc: support software ECC

With these changes the driver can be used with software BCH ECC which
is useful for NAND chips that require a stronger ECC than the i.MX
hardware supports.

The controller normally interleaves user data with OOB data when
accessing the NAND chip. With Software BCH ECC we write the data
to the NAND in a way that the raw data on the NAND chip matches the
way the NAND layer sees it. This way commands like NAND_CMD_RNDOUT
work as expected.

This was tested on i.MX27 but should work on the other SoCs supported
by this driver as well.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240522-mtd-nand-mxc-nand-exec-op-v4-3-75b611e0ac44@pengutronix.de