defconfig: msm: Enable PCIe EP and MHI device on sdxprairie

Modem host interface (MHI) device driver is used by clients
to communicate over PCIe for data transfers. Clients include
MBIM that uses MHI to setup a data call and initiate data
transfers between host and device.

Change-Id: I9f40096e06517cf1c5f95e5ac618ffcdf455488b
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>