ARM: Kirkwood: enable GPIO fan alarm support for 2Big Network v2

On the LaCie 2Big Network v2 (net2big_v2) board, the fan alarm is not
wired to the I2C fan controller but to a separe GPIO. This GPIO can be
controlled by using the gpio-fan driver.

This patch adds the gpio-fan alarm description in the net2big_v2 DTS.
Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/kirkwood-net2big.dts

@@ -27,6 +27,11 @@ memory {
 		device_type = "memory";
 		reg = <0x00000000 0x10000000>;
 	};
+
+	fan {
+		compatible = "gpio-fan";
+		alarm-gpios = <&gpio0 25 GPIO_ACTIVE_LOW>;
+	};
 };
 
 &regulators {