mtd: spinand: Move ECC related definitions earlier in the driver

Prepare the creation of a SPI-NAND on-die ECC engine by gathering the
ECC-related code earlier enough in the core to avoid the need for
forward declarations.

The next step is to actually create that engine by implementing the
generic ECC interface.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200930154109.3922-3-miquel.raynal@bootlin.com