[media] em28xx: add support for GPO controlled analog capturing LEDs

Some devices are equipped with a capturing status LED that needs to be
switched on/off explicitly via a GPO port.

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