ARM: dts: BCM5301X: Provide defaults ports container node

Provide an empty 'ports' container node with the correct #address-cells
and #size-cells properties. This silences the following warning:

arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dt.yaml:
ethernet-switch@18007000: 'oneOf' conditional failed, one must be fixed:
        'ports' is a required property
        'ethernet-ports' is a required property
        From schema:
Documentation/devicetree/bindings/net/dsa/b53.yaml

Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>