defconfig: mdm: Enable MHI device for SDX20

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 device for SDX20.

Change-Id: I40c8b187cd60a51cb7f421b28b8e396cbee99df0
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>