UPSTREAM: leds: multicolor: Introduce a multicolor class definition

Introduce a multicolor class that groups colored LEDs
within a LED node.

The multicolor class groups monochrome LEDs and allows controlling two
aspects of the final combined color: hue and lightness. The former is
controlled via the intensity file and the latter is controlled
via brightness file.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
[squashed leds: multicolor: Fix camel case in documentation in]

Bug: 260685629
(cherry picked from commit 55d5d3b46b08a4dc0b05343d24640744e7430ed7)
Change-Id: Ib1f41d74ace8e3a9c1071d52202c8d8b70a912e0
Signed-off-by: Farid Chahla <farid.chahla@sony.com>