media: i2c: imx290: Add exposure time control

Support configuring the exposure time, which is expressed as the
complement of the exposure time (frame period minus integration time).
The frame period is currently fixed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>