hdmitx: add auto low latency mode(ALLM) [1/1]

hdmitx: add auto low latency mode(ALLM) [1/1]

PD#SWPL-4705

Problem:
Lack ALLM function

Solution:
Add ALLM function

Verify:
GXL/P212
If Rx supports ALLM, then
echo 1 > /sys/class/amhdmitx/amhdmitx0/allm_mode
otherwise it will set failed, cat allm_mode and will get 0.

Change-Id: I00233e5a5aac133b405590e7df78c7c4805ed0ef
Signed-off-by: Zongdong Jiao <zongdong.jiao@amlogic.com>