dt-bindings: mtd: Document nand-ecc-engine

This property is needed to precisely point to the hardware ECC engine
to use when there are several of them available. Here, hardware also
refers to the on-die possibility.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-7-miquel.raynal@bootlin.com

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

@@ -55,6 +55,19 @@ patternProperties:
         $ref: /schemas/types.yaml#/definitions/string
         enum: [none, soft, hw, hw_syndrome, hw_oob_first, on-die]
 
+      nand-ecc-engine:
+        allOf:
+          - $ref: /schemas/types.yaml#/definitions/phandle
+        description: |
+          A phandle on the hardware ECC engine if any. There are
+          basically three possibilities:
+          1/ The ECC engine is part of the NAND controller, in this
+          case the phandle should reference the parent node.
+          2/ The ECC engine is part of the NAND part (on-die), in this
+          case the phandle should reference the node itself.
+          3/ The ECC engine is external, in this case the phandle should
+          reference the specific ECC engine node.
+
       nand-ecc-placement:
         allOf:
           - $ref: /schemas/types.yaml#/definitions/string