[media] em28xx: add support for illumination button and LED

The SpeedLink VAD Laplace webcam is equipped with an illumination button and
an illumination LED. When the button is pressed, the driver must toggle the
LED state via the corresponding GPO port.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>