mhi_dev: mhi: Add MHI device driver

The Modem Host Interface (MHI) device driver supports
clients to send control and data packets such as
IP data packets, control messages and diagnostic
data between the host and the device. It follows the
MHI specification to transfer data.

The driver interfaces with the IPA driver for hardware
accelerated channels and PCIe endpoint driver to
communicate between the host and the device.

The driver exposes to both userspace and kernel space
generic IO read/write/open/close system calls and
kernel APIs to communicate and transfer data between
the host and the device.

Change-Id: I369a0ab198b17d3d8d043836397cecde7ba7dbd3
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>