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

This property has been extensively used for almost two decades already,
a lot of device trees use it, this is not the preferred way to configure
the ECC engines but we cannot just ignore it. Describe the property,
list the exact strings which have once been supported and mark it
deprecated.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-5-miquel.raynal@bootlin.com