dt-bindings: leds: class-multicolor: reference class directly in multi-led node

The leds/common.yaml is referenced directly in each LED node, which
leads to people doing the same with leds/leds-class-multicolor.yaml.
This is not correct because leds-class-multicolor.yaml defined multi-led
property and its children.  Some schemas implemented this incorrect.

Rework this to match same behavior common.yaml, so expect the multi-led
node to reference the leds-class-multicolor.yaml.  Fixing allows to add
unevaluatedProperties:false.

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