drm/msm/dsi-staging: add dsi definitions for new dsi driver

Add header with definitions which are common to all
dsi driver components.

Change-Id: Ic460f81ba11438c7f0032303824144cc913f20fd
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Krishna Srinivas Kundurthi <kskund@codeaurora.org>