drivers: led: add leds-group-multicolor driver

This driver supports multi-color LEDs built with several monochromatic
LEDs. ->set_color is the only LED driver API method implemented.

Instead of calling led_set_brightness() for each monochromatic LED,
led_set_color() can be called on the leds-group-multicolor device to
set all colors at once.

See the leds-group-multicolor DT binding for details.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>