media: uapi: HEVC: Add SEI pic struct flags

The possible values for the field_pic field in the v4l2_hevc_dpb_entry
structure are defined in the table D.2 in HEVC specification section D.3.3.
Add flags and documentation for each of them.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>