dt-bindings: net: dsa: Require ports or ethernet-ports

Bindings using dsa.yaml#/$defs/ethernet-ports specify that
a DSA switch node need to have a ports or ethernet-ports
subnode, and that is actually required, so add requirements
using oneOf.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>