media: bcm2835-unicam: Add support for the FRAME_SYNC event

The FRAME_SYNC event is useful for userspace image processing algorithms
to program the camera sensor as early as possible after frame start.
Support it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>