hw/misc/led: Allow connecting from GPIO output

hw/misc/led: Allow connecting from GPIO output

Some devices expose GPIO lines.

Add a GPIO qdev input to our LED device, so we can
connect a GPIO output using qdev_connect_gpio_out().

When used with GPIOs, the intensity can only be either
minium or maximum. This depends of the polarity of the
GPIO (which can be inverted).
Declare the GpioPolarity type to model the polarity.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Message-Id: <20200912134041.946260-3-f4bug@amsat.org>