ath79: Add LED labels for Airtight C-75

Keep labels since OpenWrt userland tooling (get_dt_led) depends on them
to find the LED instances referenced by the led-* aliases.

The label for the amber power LED was removed in 4eefdc7.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>

target/linux/ath79/dts/qca9550_airtight_c-75.dts

@@ -32,12 +32,14 @@
 		compatible = "gpio-leds";
 
 		led_power_amber: power-amber {
+			label = "amber:power";
 			color = <LED_COLOR_ID_AMBER>;
 			function = LED_FUNCTION_POWER;
 			gpios = <&gpio 14 GPIO_ACTIVE_LOW>;
 		};
 
 		led_power_green: power-green {
+			label = "green:power";
 			color = <LED_COLOR_ID_GREEN>;
 			function = LED_FUNCTION_POWER;
 			gpios = <&gpio 4 GPIO_ACTIVE_LOW>;