dt-bindings: mtd: Create a file for raw NAND chip properties

In an effort to constrain as much as we can the existing binding, we
want to add "unevaluatedProperties: false" in all the NAND chip
descriptions part of NAND controller bindings. But in order to do that
properly, we also need to reference a file which contains all the
"allowed" properties. Right now this file is nand-chip.yaml but in
practice raw NAND controllers may use additional properties in their
NAND chip children node. These properties are listed under
nand-controller.yaml, which makes the "unevaluatedProperties" checks
fail while the description are valid. We need to move these NAND chip
related properties into another file, because we do not want to pollute
nand-chip.yaml which is also referenced by eg. SPI-NAND devices.

Let's create a raw-nand-chip.yaml file to reference all the properties a
raw NAND chip description can contain. The chain of inheritance becomes:
  nand-controller.yaml <- raw-nand-chip.yaml
  raw-nand-chip.yaml   <- nand-chip.yaml
  spi-nand.yaml        <- nand-chip.yaml

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-3-miquel.raynal@bootlin.com