lantiq: move mux for pins into subnodes

The mux need to be defined in a subnode to be considered by the pinctrl
framework. These muxes aren't set as expected and might cause not
working subsystems.

Fixes: 8e7b573b ("lantiq: dts: assign the PCI pins to the PCI controller node")
Fixes: dcb5e522 ("lantiq: dts: assign the STP pins to the STP GPIO controller node")
Fixes: 660200e5 ("lantiq: dts: assign the GPHY LED pins to the Ethernet controller node")
Signed-off-by: Mathias Kresin <dev@kresin.me>