[media] Add documentation for V4L2_PIX_FMT_VP9

Add documentation for V4L2_PIX_FMT_VP9.

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>