dt-bindings: mtd: Mark nand-ecc-placement deprecated

The nand-ecc-placement property has been deprecated for a long time
already, it does not really mean something useful for the ECC engines
and is anyway in the vast majority of cases totally useless. Just mark
it deprecated to avoid appealing people to use it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-4-miquel.raynal@bootlin.com