ARM: dts: exynos: fix PWM LED max brightness on Odroid XU4

There is no "max_brightness" property as pointed out by dtschema:

  arch/arm/boot/dts/exynos5422-odroidxu4.dt.yaml: led-controller: led-1: 'max-brightness' is a required property

Fixes: 66583560 ("ARM: dts: Add support Odroid XU4 board for exynos5422-odroidxu4")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210505135941.59898-5-krzysztof.kozlowski@canonical.com

arch/arm/boot/dts/exynos5422-odroidxu4.dts

@@ -24,7 +24,7 @@ led-1 {
 			label = "blue:heartbeat";
 			pwms = <&pwm 2 2000000 0>;
 			pwm-names = "pwm2";
-			max_brightness = <255>;
+			max-brightness = <255>;
 			linux,default-trigger = "heartbeat";
 		};
 	};