dt-bindings: net: dsa: allow additional ethernet-port properties

Explicitly allow additional properties for both the ethernet-port and
ethernet-ports properties. This specifically will allow the qca8k.yaml
binding to use shared properties.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>