drivers: led: introduce led_set_color syscall

The led_set_color syscall adds support for multicolor (i.e.
multi-channel) LEDs to the LED API. It allows a user to set all the
colors/channels of a LED at once by passing a color array. Note that
this array must provide an entry per color/channel and must also be
ordered following the color mapping of the LED. This color mapping can
be either retrieved from the "color-mapping" DT property or from the LED
driver itself (by using the led_get_info syscall).

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