ARM: dts: msm: Add MHI device for sdxpoorwills

MHI (Modem host interface) device driver is used by clients
to transfer data between host and device over PCIe.

Change-Id: Ib59058ac4620401f17704fe9c0d61473838b43d5
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>