dt-bindings: net: add generic ethernet-switch-port binding

The dsa-port.yaml binding had several references that can be common to all
ethernet ports, not just dsa-specific ones. Break out the generic bindings
to ethernet-switch-port.yaml they can be used by non-dsa drivers.

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>