ramips/leds: use devicetree alias led_wlan

ramips/leds: use devicetree alias led_wlan

Similiar to the lantiq target use a dts alias to define the wlan led
instead of static mapping in /etc/board.d/01_leds. Reduce code
duplication.
A device tree must define the alias "led-wlan" similiar to "led-boot".

/ {
        aliases {
                led-wlan = &led_wlan;
        };

        [..]

        led_wlan: wlan {
            label = "tl-wr841n-v14:green:wlan";
            gpios = <&gpio1 9 GPIO_ACTIVE_LOW>;
        };
};

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>