ARM: dts: msm: Add MHI device for MDM9640

Enable MHI device to support communication between
host and device over PCIe. Clients include QTI
for software channels and IPA for hardware accelerated
channels.

Change-Id: I07c1af28c6f4f5c14c2f3e5088f9742b23835f58
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>