ARM: dts: msm: add property for DSI trigger channel on SDM670

On SDM670 platform nt36850 panel only supports sending commands
from DSI1(right side) controller. Add a property to mention the
same.

Change-Id: I88558f615a08b6881be7c778beadb019565e0835
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>