mtd: spinand: Use the external ECC engine logic

Now that all the logic is available in the NAND core, let's use it
from the SPI-NAND core. Right now there is no functional change as the
default ECC engine for SPI-NANDs is set to 'on-die', but user can now
use software correction if they want to by just setting the right
properties in the DT.

Also note that the OOB layout handling is removed from the SPI-NAND
core as each ECC engine is supposed to handle it by it's own; users
should not be aware of that.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201001102014.20100-4-miquel.raynal@bootlin.com