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: Ie9843d5e014f97ff1d75c9973e38414d0bf9344f
Signed-off-by: Dilip Kota <dkota@codeaurora.org>