dt-bindings: net: freescale: enetc: Add connection bindings for ENETC ethernet nodes

Define connection bindings (external PHY connections and internal links)
for the ENETC on-chip ethernet controllers.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>