msm: mhi_dev: Add MHI driver snapshot

Modem host interface (MHI) driver is used by clients
to transfer data between host and device over PCIe
end point driver. Software clients include QTI and
software IP to setup data call and initiate data
transfer. Driver interfaces with IPA, end point PCIe
for data transfer. Add a callback for kernel clients
to receive notification once the software channel
for which the client has registered has opened/closed.

This diff is taken as of msm-3.18
'commit <18d7487> ("Merge "msm: ipa: Fix to check only
reset IPA stats can have data as NULL"")'.

Change-Id: I3dd54121d7dde6ddc6138da8d2ab9802452d4681
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>