dt-bindings: mtd: Clarify all partition subnodes

Over time the various ways to define MTD partitions has evolved. Most of
the controllers support several different bindings. Let's define all
possible choices in one file and mark the legacy ones deprecated. This
way, we can just reference this file and avoid dupplicating these
definitions.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-2-miquel.raynal@bootlin.com