dt-bindings: leds: pwm-multicolor: document max-brigthness

The Multicolor PWM LED uses max-brigthness property (in the example and
in the driver), so document it to fixi dt_binding_check warning like:

  leds/leds-pwm-multicolor.example.dtb:
    led-controller: multi-led: Unevaluated properties are not allowed ('max-brightness' was unexpected)

Reported-by: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>