ARM: dts: exynos: fix PWM LED max brightness on Odroid XU/XU3

There is no "max_brightness" property.  This brings the intentional
brightness reduce of green LED and dtschema checks as well:

  arch/arm/boot/dts/exynos5410-odroidxu.dt.yaml: led-controller-1: led-1: 'max-brightness' is a required property

Fixes: 719f39fe ("ARM: dts: exynos5422-odroidxu3: Hook up PWM and use it for LEDs")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210505135941.59898-3-krzysztof.kozlowski@canonical.com