dt-bindings: mt76: add active-low property for led

LEDs can be in low-active mode, driver already supports it, but
documentation is missing. Add documentation for the dt property.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>