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