hdmitx: add SCDC/CED function [1/1]

hdmitx: add SCDC/CED function [1/1]

PD#SWPL-6361

Problem:
Lack SCDC/CED function to check Rx TMDS status

Solution:
Add SCDC/CED function
By default, this function is not open.
Enable it in board dts file like below:
  &amhdmitx {
	cedst_en = <1>;
  };
Also, you can manually enable it by
  'echo 1 > /sys/class/amhdmitx/amhdmitx0/cedst_policy'
Then listen '/sys/class/extcon/hdmi_cedst/uevent'
and check 'cat /sys/class/amhdmitx/amhdmitx0/cedst_count'

Verify:
G12/U212

Change-Id: Ic9c90936bad643ea95d418d7b019eb37210d7123
Signed-off-by: Zongdong Jiao <zongdong.jiao@amlogic.com>