dt-bindings: mtd: Document nand-ecc-engine

This property is needed to precisely point to the hardware ECC engine
to use when there are several of them available. Here, hardware also
refers to the on-die possibility.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-7-miquel.raynal@bootlin.com