video: Add comments to struct sandbox_sdl_plat

This struct is not commented but needs it. Also fix the comment in
check_vidconsole_output() about the encoding for the rotation value.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
diff --git a/test/dm/video.c b/test/dm/video.c
index 68f5ba4..729c31b 100644
--- a/test/dm/video.c
+++ b/test/dm/video.c
@@ -188,7 +188,8 @@
  * check_vidconsole_output() - Run a text console test
  *
  * @uts:	Test state
- * @rot:	Console rotation (0, 90, 180, 270)
+ * @rot:	Console rotation (0=normal orientation, 1=90 degrees clockwise,
+ *		2=upside down, 3=90 degree counterclockwise)
  * @wrap_size:	Expected size of compressed frame buffer for the wrap test
  * @scroll_size: Same for the scroll test
  * @return 0 on success