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

drivers/mtd/mtdcore.c

@@ -531,6 +531,7 @@ static int mtd_nvmem_reg_read(void *priv, unsigned int offset,
 
 static int mtd_nvmem_add(struct mtd_info *mtd)
 {
+	struct device_node *node = mtd_get_of_node(mtd);
 	struct nvmem_config config = {};
 
 	config.id = -1;
@@ -543,7 +544,7 @@ static int mtd_nvmem_add(struct mtd_info *mtd)
 	config.stride = 1;
 	config.read_only = true;
 	config.root_only = true;
-	config.no_of_node = true;
+	config.no_of_node = !of_device_is_compatible(node, "nvmem-cells");
 	config.priv = mtd;
 
 	mtd->nvmem = nvmem_register(&config);