lantiq: xrx200: use vlan for ethernet wan port

Using the lantiq,wan device tree property for one interface node and
the lantiq,switch device tree property for another interface node at
the same time was never intended/isn't supported at the moment.

The property is meant to be used in two phy operation mode where one
phy is assigned to an interface without lantiq,* device tree property
and the other phy is assigned to an interface with the lantiq,wan
device property to have two netdevs.

If both properties are used at the same time, the lantiq,wan interface
is shown as independent netdev but not able to operate independent. The
port needs to be managed via swconfig. These dependency is not obvious
and fooled already a lot of users.

Add a default WAN vlan for xrx200 devices having an ethernet WAN port
and remove the lantiq,wan device tree property. Leave it up to the user
to set the ethernet WAN port as default WAN interface or to use this
port as additional LAN port.

Signed-off-by: Mathias Kresin <dev@kresin.me>