media: bcm2835-unicam: Add option for a GPIO to reflect FS/FE timing

The legacy stack had an option to have a GPIO track frame start and
end events to give basic synchronisation to the incoming image stream.
https://forums.raspberrypi.com/viewtopic.php?t=190314



Replicate this in the kernel Unicam driver.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>