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 <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/admin-guide/media/cec.rst

···
   you can control the CEC line through this driver. This supports error
   injection as well.
 
+- cec-gpio and Allwinner A10 (or any other driver that uses the CEC pin
+  framework to drive the CEC pin directly): the CEC pin framework uses
+  high-resolution timers. These timers are affected by NTP daemons that
+  speed up or slow down the clock to sync with the official time. The
+  chronyd server will by default increase or decrease the clock by
+  1/12th. This will cause the CEC timings to go out of spec. To fix this,
+  add a 'maxslewrate 40000' line to chronyd.conf. This limits the clock
+  frequency change to 1/25th, which keeps the CEC timings within spec.
+
 
 Utilities
 =========