atv_demod: Fix ATV audio output noise when switching channels to play [1/1]

atv_demod: Fix ATV audio output noise when switching channels to play [1/1]

PD#SWPL-2297, PD#SWPL-5450, PD#TV-2311

Problem:
Fix ATV audio output noise when switching channels to play.

Solution:
1.Add the judgment of SNR to prevent the weak signal
  from entering the OV mode.
2.Wait for the signal to be completely stable before OV detection.
3.OV detection is turned off by default,
  and will be turned on again when non-standard signals are encountered.
4.The OV detection moves from afc timer to monitor.
5.Optimize detection of A2/FM.
6.atv demod ver: V2.08.

Verify:
verified by x301

Change-Id: I3064230472ce51e1192a2d08ef6cc6f4ba10abaa
Signed-off-by: nengwen.chen <nengwen.chen@amlogic.com>