dt-bindings: drm: msm: add bond mode option

Add bond mode option to specify which two or three DP controllers
can work in bond mode.

Change-Id: I26b0fedd3d861deafae76f6bdda7bddc74ac5363
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Jim Wang <jimwan@codeaurora.org>