drm/msm/dp: add dp-mst sideband simulator

Add sideband simulator aux bridge to re-direct sideband message
from aux channel to internal emulation software stack. When this
bridge is connected to DP device, simulator will read ports configuration
from device tree and fake all sideband messages.

Change-Id: Ib8af2e0f4ba9b0d02413d40b796e5006b376ada8
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>