dt-bindings: mtd: partitions: Constrain the list of parsers

Parser compatibles cannot be used anywhere, and the list is limited. In
order to constrain this list, enumerate them all under the top
"partitions" subnode. New parsers will have to add their own compatible
here as well.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-12-miquel.raynal@bootlin.com