dt-bindings: net: Add tsnep Ethernet controller

The TSN endpoint Ethernet MAC is a FPGA based network device for
real-time communication.

It is integrated as normal Ethernet controller with
ethernet-controller.yaml and mdio.yaml.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>