dt-bindings: mtd: onenand: Mention the expected node name

The chip node name in this driver is expected to be different and should
be prefixed with onenand instead of the regular "flash" string, so
mention it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-8-miquel.raynal@bootlin.com