leds: pwm-multicolor: Support active-low LEDs

Add support for LEDs wired up to light when the PWM output is low, just
like the regular PWM LEDs.

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>