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

This adds the binding for the GSWIP (Gigabit switch) core found in the
xrx200 / VR9 Lantiq / Intel SoC.

This part takes care of the switch, MDIO bus, and loading the FW into
the embedded GPHYs.

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