Add a LED device which can be connected to a GPIO output.
They can also be dimmed with PWM devices. For now we do
not implement the dimmed mode, but in preparation of a
future implementation, we start using the LED intensity.

LEDs are limited to a fixed set of colors.

Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200912134041.946260-2-f4bug@amsat.org>