leds: rgb: Add a multicolor LED driver to group monochromatic LEDs

Grouping multiple monochrome LEDs into a multicolor LED device has a few
benefits over handling the group in user-space:
- The state of the LEDs relative to each other is consistent. In other
  words, if 2 threads competes to set the LED to green and red, the
  end-result cannot be black or yellow.
- The multicolor LED as a whole can be driven through the sysfs LED
  interface.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Reviewed-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230728153731.3742339-5-jjhiblot@traphandler.com


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