msm: mhi: Add MHI core driver

Enable the MHI core driver for communication
between host and device using PCIe as an interconnect
and supporting MHI as the communication protocol.

The driver exposes several kernel space APIs
for use by other kernel entities to interface to
the PCIe device over MHI.

APIs for read and write and other notifications
are supported by this MHI driver.

Support for full power management and device reset
is also included.

CRs-Fixed: 689329
Change-Id: Ibc2fd7c2d5689001485f71b1133ada2c4ca236a9
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>