dt-bindings: net: dsa: sf2: fix brcm,use-bcm-hdr documentation

The property use-bcm-hdr was documented as an entry under the ports node
for the bcm_sf2 DSA switch. This property is actually evaluated for each
port. Correct the documentation to match the actual behavior and properly
reference dsa-port.yaml for additional properties of the node.
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/brcm,sf2.yaml

@@ -85,11 +85,16 @@ properties:
   ports:
     type: object
 
-    properties:
-      brcm,use-bcm-hdr:
-        description: if present, indicates that the switch port has Broadcom
-          tags enabled (per-packet metadata)
-        type: boolean
+    patternProperties:
+      '^port@[0-9a-f]$':
+        $ref: dsa-port.yaml#
+        unevaluatedProperties: false
+
+        properties:
+          brcm,use-bcm-hdr:
+            description: if present, indicates that the switch port has Broadcom
+              tags enabled (per-packet metadata)
+            type: boolean
 
 required:
   - reg