dt-bindings: net: dsa: inherit the ethernet-controller DT schema

Since a switch is basically a bunch of Ethernet controllers, just
inherit the common schema for one to get stronger type validation of the
properties of a port.

For example, before this change it was valid to have a phy-mode = "xfi"
even if "xfi" is not part of ethernet-controller.yaml, now it is not.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>