ARM: dts: msm: Enable MHI on SDX20 PCIe EP platform

Modem host device (MHI) driver is used to communicate
between host and device over PCIe. Clients include
QTI for software channels and IPA for hardware
accelerated channels. Enable MHI node for SDX20.

Change-Id: If3466ff647d7c0435bce4ffd9150d6fc21148203
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>