dt-bindings: mtd: partitions: support label/name only partition

Document new partition nodes that declare only the label/name instead
of the reg used to provide an OF node for partition registred at runtime
by parsers. This is required for nvmem system to declare and detect
nvmem-cells.

With these special partitions, the reg / offset is not required and a
'partition-' prefix is needed.
The node name with the 'partition-' prefix stripped, is used to match
the partition allocated by the parser at runtime and the parser will
provide reg and offset of the mtd.
If the partition to match contains invalid char for a node name, the
label binding can be used to declare the partition name.

NVMEM will use the data from the parser and provide the NVMEM cells
declared in the DTS, "connecting" the dynamic partition with a
static declaration of cells in them.

Signed-off-by: Christian Marangi <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/20220622010628.30414-2-ansuelsmth@gmail.com