drm/msm/dp: add support for dp aux

Add AUX communication related functionalities that a client
can use. Provide a get API that a client can call to get
access to AUX module's functionalities. The AUX functionalities
includes read and write data from and to a sink device. This
module can be used to read EDID, DPCD, link training data and
HDCP related data.

Change-Id: I97fed02216733e6cccdd00b79e35d7b9976241af
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>