video: Add a comment for struct video_uc_platdata

Add a few notes to explain the purpose of each member of this struct.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/video.h b/include/video.h
index e7c58e8..813b565 100644
--- a/include/video.h
+++ b/include/video.h
@@ -19,6 +19,18 @@
 
 struct udevice;
 
+/**
+ * struct video_uc_platdata - uclass platform data for a video device
+ *
+ * This holds information that the uclass needs to know about each device. It
+ * is accessed using dev_get_uclass_platdata(dev). See 'Theory of operation' at
+ * the top of video-uclass.c for details on how this information is set.
+ *
+ * @align: Frame-buffer alignment, indicating the memory boundary the frame
+ *	buffer should start on. If 0, 1MB is assumed
+ * @size: Frame-buffer size, in bytes
+ * @base: Base address of frame buffer, 0 if not yet known
+ */
 struct video_uc_platdata {
 	uint align;
 	uint size;