dt-bindings: net: dsa: Document dsa-tag-protocol property

The 'dsa-tag-protocol' is used to force a switch tree to use a
particular tag protocol, typically because the Ethernet controller
that it is connected to is not compatible with the default one.
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/dsa.yaml

@@ -70,6 +70,15 @@ patternProperties:
               device is what the switch port is connected to
             $ref: /schemas/types.yaml#/definitions/phandle
 
+          dsa-tag-protocol:
+            description:
+              Instead of the default, the switch will use this tag protocol if
+              possible. Useful when a device supports multiple protcols and
+              the default is incompatible with the Ethernet device.
+            enum:
+              - dsa
+              - edsa
+
           phy-handle: true
 
           phy-mode: true