dt-bindings: mtd: mediatek,nand-ecc-engine: Add compatible for MT7986

Add dt-bindings documentation of ECC for MediaTek MT7986 SoC
platform.

Signed-off-by: Xiangsheng Hou <xiangsheng.hou@mediatek.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230201021500.26769-5-xiangsheng.hou@mediatek.com