dt-bindings: net: dsa: xrs700x: add missing CPU port phy-mode to example

Judging by xrs700x_phylink_get_caps(), I deduce that this switch
supports the RGMII modes on port 3, so state this phy-mode in the
example, such that users are encouraged to not rely on avoiding phylink
for this port.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/dsa/arrow,xrs700x.yaml

@@ -63,6 +63,8 @@ examples:
                     reg = <3>;
                     label = "cpu";
                     ethernet = <&fec1>;
+                    phy-mode = "rgmii-id";
+
                     fixed-link {
                         speed = <1000>;
                         full-duplex;