dt-bindings: mtd: Argue in favor of keeping additionalProperties set to true

In most cases we try to avoid it but in some cases this is
needed. Clarify why by adding a small comment.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-16-miquel.raynal@bootlin.com