media: Documentation: admin-guide: cec.rst: document NTP issue

The CEC pin framework is affected by NTP daemons speeding up or slowing
down the system clock. Document this and explain how to fix this for
chronyd.

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