slim-msm: Synchronize SSR callbacks

Subsystem will restart within short timeframe.
Synchronise subsytem up/down callback notifications
to avoid functionality failures.
Use mutex locks to achieve synchronization.

Change-Id: I5881c7d468507bb8402a2e9f8178b9c31e57e8a5
Signed-off-by: Dilip Kota <dkota@codeaurora.org>