dt-bindings: net: Add lantiq, xrx200-net DT bindings

This adds the binding for the PMAC core between the CPU and the GSWIP
switch found on the xrx200 / VR9 Lantiq / Intel SoC.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: devicetree@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>