ramips: add LED trigger for TL-WR902AC v3 WAN LED

This adds an LED trigger for the WAN LED on top of the TP-Link
TL-WR902AC v3. Currently, only the LED on the port itself shows the link
state, while the LED on top of the device stays dark.

The WAN port of the device is a hybrid LAN/WAN one, hence why the LED at
the port was labled LAN.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit c48b571ad7)

target/linux/ramips/base-files/etc/board.d/01_leds

@@ -360,6 +360,7 @@ tplink,tl-wr842n-v5)
 tplink,tl-wr902ac-v3)
 	ucidef_set_led_wlan "wlan2g" "wlan2g" "$boardname:green:wlan" "phy0tpt"
 	ucidef_set_led_switch "lan" "lan" "$boardname:green:lan" "switch0" "0x10"
+	ucidef_set_led_switch "wan" "wan" "$boardname:green:wan" "switch0" "0x10"
 	;;
 u25awf-h1)
 	set_wifi_led "u25awf:red:wifi"