dt-bindings: net: add generic ethernet-switch

The dsa.yaml bindings had references that can apply to non-dsa switches. To
prevent duplication of this information, keep the dsa-specific information
inside dsa.yaml and move the remaining generic information to the newly
created ethernet-switch.yaml.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.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>