leds: core: Introduce led_get_color_name() function

This is similar to the existing led_colors[] array but is safer to use and
usable by everyone.

Getting string representations of color ids is useful for drivers
which are handling color IDs anyways, for example for the multicolor API.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240613-cros_ec-led-v3-1-500b50f41e0f@weissschuh.net


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