dt-bindings: net: dsa: split generic port definition from dsa.yaml

Some switch may require to add additional binding to the node port.
Move DSA generic port definition to a dedicated yaml to permit this.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>