dt-bindings: net: dsa: rzn1-a5psw: add missing CPU port phy-mode to example

To prevent warnings during "make dt_bindings_check" after dsa-port.yaml
will make phylink properties mandatory, add phy-mode = "internal" to the
example.

This new property is taken straight out of the SoC dtsi at
arch/arm/boot/dts/r9a06g032.dtsi, so it seems likely that only the
example needs to be fixed, rather than DT blobs in circulation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>