dt-bindings: leds: pwm-multicolour: re-allow active-low

active-low was lifted to the common schema for leds, but it went
unnoticed that the leds-multicolour binding had "additionalProperties:
false" where the other users had "unevaluatedProperties: false", thereby
disallowing active-low for multicolour leds. Explicitly permit it again.

Fixes: c94d1783

 ("dt-bindings: net: phy: Make LED active-low property common")
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>