mhi_dev: add MHI device driver snapshot

Snapshot of PCIe MHI device driver as of msm-4.14.
'commit <adeff59> (""drivers: clk: msm:
fix compilation for non DEBUG_FS build"")'.

Fix several warnings as part of propagation to 5.4.

Change-Id: I48b88d8e8900cb6e166c24ad2322a6fa2ec6fb8a
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>