libcamera: camera_sensor: Prepare for multi-subdev camera sensors

While most camera sensors are exposed to userspace as a single subdev,
this is not always the case. A notable exception is the smiapp driver,
which exposes two or three subdevs. The subdev at the output of the
camera sensor will thus have more than one pad. Prepare for this by
removing the single-pad restriction, and locating the source pad
dynamically.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/include/camera_sensor.h

@@ -70,6 +70,8 @@ protected:
 private:
 	const MediaEntity *entity_;
 	std::unique_ptr<V4L2Subdevice> subdev_;
+	unsigned int pad_;
+
 	std::string model_;
 
 	std::vector<unsigned int> mbusCodes_;