dt-bindings: leds: Add multicolor PWM LED bindings

[ Upstream commit ac123741 ]

This allows to group multiple PWM-connected monochrome LEDs into
multicolor LEDs, e.g. RGB LEDs.

Signed-off-by: Sven Schwermer <sven.schwermer@disruptive-technologies.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Stable-dep-of: 609bc99a ("dt-bindings: leds: class-multicolor: Fix path to color definitions")
Signed-off-by: Sasha Levin <sashal@kernel.org>