dt-bindings: mtd: Argue in favor of keeping additionalProperties set to true

In most cases we try to avoid it but in some cases this is
needed. Clarify why by adding a small comment.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-16-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mtd/mtd.yaml

@@ -60,6 +60,7 @@ patternProperties:
     required:
       - compatible
 
+# This is a generic file other binding inherit from
 additionalProperties: true
 
 examples:

Documentation/devicetree/bindings/mtd/nand-chip.yaml

@@ -70,4 +70,5 @@ properties:
 required:
   - reg
 
+# This file can be referenced by more specific devices (like spi-nands)
 additionalProperties: true

Documentation/devicetree/bindings/mtd/nand-controller.yaml

@@ -129,6 +129,7 @@ required:
   - "#address-cells"
   - "#size-cells"
 
+# This is a generic file other binding inherit from and extend
 additionalProperties: true
 
 examples:

Documentation/devicetree/bindings/mtd/partitions/partition.yaml

@@ -64,4 +64,5 @@ then:
     $nodename:
       pattern: '^partition-.*$'
 
+# This is a generic file other binding inherit from and extend
 additionalProperties: true