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>