The Samsung CSIS MIPI receiver provides a start-of-frame interrupt and
a framecount register. As the CSI receiver is the hardware unit that lies
closest to the sensor, the frame counter is the best we can get on these
devices. In case of the ISI available on the i.MX8 M Plus it is also the
only native start-of-frame signal available.

This patch exposes the sof interrupt and the framecount as
V4L2_EVENT_FRAME_SYNC event on the subdevice.

It was tested on a Debix-Som-A with a 6.8-rc4 kernel.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Link: https://lore.kernel.org/r/20240314093652.56923-1-stefan.klug@ideasonboard.comSigned-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>