dt-bindings: mtd: nand-controller: Harmonize the property types

Harmonize the different properties in this file by:
* dropping the non-necessary allOf's
* always defining the keywords in the following order:
    - first the "description" (when relevant),
    - then the "type"/"$ref" and the other generic keywords ("enum",
      "default", etc).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-4-miquel.raynal@bootlin.com