power: bcl: Serialize frequency mitigation request from BCL events

For cpu frequency mitigation, there is one common device request
handle for all BCL events. There is a possible race condition for
frequency mitigation request if two BCL events are cleared
simultaneously and cpu frequency mitigation handler for these two
events are handled in different cpus. This gives rise to a scenario
where cpu frequency in mitigated state even though all BCL events
are cleared.
To avoid this race condition, serialize frequency mitigation request
using a mutex.

CRs-Fixed: 2005131
Change-Id: I29b189b2ecd18603441d0b69faa4c38123244a4a
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>