dt-bindings: leds: pwm-multicolor: Add active-low property

Add the active-low property to LEDs that are part of a multicolor LED
just like the regular PWM LEDs have.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Pavel Machek <pavel@ucw.cz>