dt-bindings: nvmem: drop $nodename restriction

Drop $nodename restriction as now mtd partition can also be used as
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/20210312062830.20548-2-ansuelsmth@gmail.com