media: uapi: h264: Add DPB entry field reference flags

Using the field information attached to v4l2 buffers is not enough to
determine the type of field referenced by a DPB entry: the decoded
frame might contain the full picture (both top and bottom fields)
but the reference only point to one of them.
Let's add new V4L2_H264_DPB_ENTRY_FLAG_ flags to express that.

[Keep only 2 flags and add some details about they mean]

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>