dt-bindings: mtd: nvmem-cells: Inherit from MTD partitions

The aim of MTD nvmem-cells is to treat MTD partitions as NVMEM
providers. Hence, MTD partition properties are valid here. Let's
reference mtd/partition.yaml which gives us a chance to drop
"additionalProperties: true" in favor of "unevaluatedProperties:
false".

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-15-miquel.raynal@bootlin.com