drivers: video: Add index field to video buffer

Introduce an index field to the video buffer structure to help track
individual buffers throughout the workflow.

This is particularly useful in scenarios where buffers are wrapped
in a pool, such as in GStreamer. The index allows efficient
identification of the currently dequeued buffer without needing to
iterate through the entire pool and compare buffer addresses.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>

Author: ngphibang

include/zephyr/drivers/video.h

@@ -149,6 +149,8 @@ struct video_buffer {
 	void *driver_data;
 	/** pointer to the start of the buffer. */
 	uint8_t *buffer;
+	/** index of the buffer, optionally set by the application */
+	uint8_t index;
 	/** size of the buffer in bytes. */
 	uint32_t size;
 	/** number of bytes occupied by the valid data in the buffer. */