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>

Documentation/devicetree/bindings/leds/leds-pwm-multicolor.yaml

@@ -22,6 +22,12 @@ properties:
     $ref: leds-class-multicolor.yaml#
     unevaluatedProperties: false
 
+    properties:
+      max-brightness:
+        description:
+          Maximum brightness possible for the LED
+        $ref: /schemas/types.yaml#/definitions/uint32
+
     patternProperties:
       "^led-[0-9a-z]+$":
         type: object