disp: msm: dsi: add a new DT property to support ext bridge hpd

Add a keystr into the connector property if wants this
external bridge to support hpd. Userspace can get this
info from the connector, and do hpd support.

Change-Id: I19e8c2684ef054fbd5c5bf5951357edb1b4ea40c
Signed-off-by: Shubham Talekar <quic_stalekar@quicinc.com>