[media] s5p-cec/st-cec: update TODOs

Update the TODOs explaining why these two drivers remain in
staging. The reason is that these drivers rely on userspace to
set the physical address, but that should come from the HDMI
output driver. This in turn needs the upcoming HDMI notifier
framework.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>