mtd: nand: Let software ECC engines be retrieved from the NAND core

Before making use of the ECC engines, we must retrieve them. Add the
boilerplate for the ones already available: software engines (Hamming
and BCH).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-21-miquel.raynal@bootlin.com