dt-bindings: net: dsa: utilize base definitions for standard dsa switches

DSA switches can fall into one of two categories: switches where all ports
follow standard '(ethernet-)?port' properties, and switches that have
additional properties for the ports.

The scenario where DSA ports are all standardized can be handled by
switches with a reference to the new 'dsa.yaml#/$defs/ethernet-ports'.

The scenario where DSA ports require additional properties can reference
'$dsa.yaml#' directly. This will allow switches to reference these standard
definitions of the DSA switch, but add additional properties under the port
nodes.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Alvin Šipraga <alsi@bang-olufsen.dk> # realtek
Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>