dt-bindings: mtd: Document boolean NAND ECC properties

Document nand-use-soft-ecc-engine and nand-no-ecc-engine properties.
The former is here to force software correction, the latter prevents
any correction to happen.

These properties (along with nand-ecc-engine) are supposed to be more
accurate than the current nand-ecc-modes wich is very misleading and
very often people think it is mandatory while the core should be
relied upon to decide which correction to handle.

nand-ecc-mode was already inacurate, but it becomes totally
problematic with setups where there are several hardware engines.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-8-miquel.raynal@bootlin.com