this adds the mediatek,led_source dts binding for Asus RT-AC1200 devices' dtsi, for correct switch LED behavior. The dts-binding is introduced in commit:65dc9e0980Without this, we only have constantly very fast blinking LEDs, which don't react on any traffic or LAN events at all. Signed-off-by: Tamas Balogh <tamasbalogh@hotmail.com> (cherry picked from commit771ea6f2e3)