leds: add RGB color option, as that is different from multicolor.

Multicolor is a bit too abstract. Yes, we can have
Green-Magenta-Ultraviolet LED, but so far all the LEDs we support are
RGB, and not even RGB-White or RGB-Yellow variants emerged.

Multicolor is not a good fit for RGB LED. It does not really know
about LED color.  In particular, there's no way to make LED "white".

Userspace is interested in knowing "this LED can produce arbitrary
color", which not all multicolor LEDs can.

Signed-off-by: Pavel Machek <pavel@ucw.cz>