dt-bindings: Add a new extern bridge config

Instead of mdss-dsi-ext-bridge-mode, this property
define an array of u32 value, this value needs match
the port reg config, we can use it to select the right
panel or bridge.

Change-Id: I0af18ffcc131b9ff7f921eebe147a230eb7f4fa9
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>