disp: msm: dsi: Add a new DT property panel test pin

This pin is a output pin from panel. Panel can
output signal of internal VSYNC and ERR_FLAG.

Change-Id: Ib8e661ca1fdb33bb7060935edb9bc1f1a858c4b3
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>