v4l2-ioctl: add VP9 format description

VP9 is a video coding format and a successor to VP8.

Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>
Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
(cherry picked from commit 60d1cd375151ee0cc1083be8a63b775cae983124)