ipc: add api to vote for upgrading thread priority

For some time critical tasks, the thread priority
might need to be upgraded to Real-Time Thread
priority level. Expose an interface from APR layer
so that clients can vote for a priority upgrade
whenever needed.

Change-Id: Ieb4afa914905750eccdf7672020a8751fdcf6462
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>