dt-bindings: mtd: Accept nand related node names

There is no addition there, but the mtd.yaml file is so generic, it can
be referenced by a wide variety of devices, including nand ones which
already define the node name to "nand@<cs>". Right now it does not lead
to any failure but when we will constrain more the schema, this will
become a problem because we want the mtd-wide properties like label or
partitions to be available for the callers.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-2-miquel.raynal@bootlin.com