[media] Add documentation for V4L2_PIX_FMT_VP9

Add documentation for V4L2_PIX_FMT_VP9.
Signed-off-by: NTiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: NWu-Cheng Li <wuchengli@chromium.org>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/pixfmt-013.rst

@@ -85,3 +85,8 @@ Compressed Formats
       - ``V4L2_PIX_FMT_VP8``
       - 'VP80'
       - VP8 video elementary stream.
+    * .. _V4L2-PIX-FMT-VP9:
+
+      - ``V4L2_PIX_FMT_VP9``
+      - 'VP90'
+      - VP9 video elementary stream.