dt-bindings: mtd: Document use of nvmem-cells compatible

Document nvmem-cells compatible used to treat mtd partitions as a
nvmem provider.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210312062830.20548-3-ansuelsmth@gmail.com