dt-bindings: mtd: Deprecate nand-ecc-mode

This property does not describe very well its purpose: it describes
the ECC engine type. Deprecate it in favor of nand-ecc-engine which
points to either the NAND part itself in case of on-die ECC, or to the
parent node in case of an integrated ECC engine in the NAND controller
(previously referred as "hardware") or to another node in case of an
external controller. Other "modes" (none/software) are achieved with
the new nand-use-soft-ecc-engine and nand-no-ecc-engine properties.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201001102014.20100-3-miquel.raynal@bootlin.com