sched: boost: Add support for nesting and priority

Multiple clients could vote for different levels of boost. Use a
bucket to count the voting of different levels and take the
one with the highest priority. It is expected that client needs to
remove their vote when finished.

Change-Id: I947c01a0e0f5e79a70fb393e61d43aa06fef7444
Signed-off-by: Runmin Wang <runminw@codeaurora.org>