lantiq: dts: assign the ASC pins to the serial controller node

lantiq: dts: assign the ASC pins to the serial controller node

Assign the ASC pins to the serial 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>