oxnas: shuttle-kd20: fix thermal zone

Fix typo ('_' vs '-') and add #cooling-cells to gpio-fan to get
thermal zone into functional state.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/oxnas/files/arch/arm/boot/dts/ox820-shuttle-kd20.dts

@@ -31,7 +31,7 @@
 		led-upgrade = &led_warn;
 	};
 
-	thermal_zones {
+	thermal-zones {
 		chassis-thermal {
 			/* Poll every 20 seconds */
 			polling-delay = <20000>;
@@ -153,6 +153,7 @@
 		gpios = <&gpio0 2 GPIO_ACTIVE_HIGH>;
 		gpio-fan,speed-map = <0    0
 				      3000 1>;
+		#cooling-cells = <2>;
 	};
 
 	gpio-poweroff {