framework, dvfs: Remove signals.

This commit removes signals and its current use
from dvfs module.

Signal functionality was introduced for use cases where events
(struct fwk_event) from one module to another module were expensive
at the run time due total number cycles it takes to allocate and
initialize on the function stack. However similar functionality
also can be achieved using smaller event objects.

This change is precursor to the change(light events) which will be
introduced in the upcoming commits.

Change-Id: I91715841a4c34ba2f251e43867b066d345d8f755
Signed-off-by: Girish Pathak <girish.pathak@arm.com>