dt-bindings: leds: Add binding for a multicolor group of LEDs

This allows to group multiple monochromatic LEDs into a multicolor
LED, e.g. RGB LEDs.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230728153731.3742339-4-jjhiblot@traphandler.com


Signed-off-by: Lee Jones <lee@kernel.org>