media: uapi: mpeg2: Split sequence and picture parameters

Typically, bitstreams are composed of a sequence header,
followed by a number of picture header and picture coding extension
headers. Each picture can be composed of a number of slices.

Let's split the MPEG-2 uAPI to follow these semantics more closely,
allowing more usage flexibility. Having these controls split up
allows applications to set a sequence control at the beginning
of a sequence, and then set a picture control for each frame.

While here add padding fields where needed, and document
the uAPI header thoroughly.

Note that the V4L2_CTRL_TYPE_{} defines had to be moved because
it clashes with existing ones. This is not really an issue
since they will be re-defined when the controls are moved
out of staging.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Tested-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>