msm: pil: Add notifications to indicate execution of proxy voting/unvoting

Add notifications to indicate that the generic work required to be
done in the proxy voting and unvoting functions has compeleted.

Moving forward, this will give drivers that have registered for
these notifications about the subsystem, a chance to do more
specific things at their end. This is crucial in moving forward
with having a generic driver for PIL and SSR across all
subsystems.

Change-Id: I8f1449ab86719ced533aa477f062d686958f50b2
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>