mtd: core: add nvmem-cells compatible to parse mtd as nvmem cells

Partitions that contains the nvmem-cells compatible will register
their direct subonodes as nvmem cells and the node will be treated as a
nvmem provider.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Tested-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210312062830.20548-1-ansuelsmth@gmail.com