dt-bindings: mtd: mediatek: Prevent NAND chip unevaluated properties

nand-on-flash-bbt is a generic property which may apply to any raw NAND
chip, it does not need to be listed in each controller
description. The raw NAND chip description file which contains the
property is already referenced, so no need to mention the property here
again.

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Xiangsheng Hou <xiangsheng.hou@mediatek.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-17-miquel.raynal@bootlin.com