media: uapi: move MPEG-2 stateless controls out of staging

Until now, the MPEG-2 V4L2 API was not exported as a public API,
and only defined in a private media header (media/mpeg2-ctrls.h).

After reviewing the MPEG-2 specification in detail, and reworking
the controls so they match the MPEG-2 semantics properly,
we can consider it ready.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
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>