media: move CEC platform drivers to a separate directory

As CEC support doesn't depend on MEDIA_SUPPORT, let's
place the platform drivers outside the media menu.

As a side effect, instead of depends on PCI, seco driver
can select it (and DMI).

Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>