lantiq: dts: assign the STP pins to the STP GPIO controller node

lantiq: dts: assign the STP pins to the STP GPIO controller node

Assign the STP pins to the STP GPIO controller node instead of using
pin hogging (where pins are assigned to the pin controller).
This is the preferred way of assigning pins upstream.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>