timer: Add a global deferrable timer

Add a global deferrable timer in addition to the per-cpu deferrable timer
to allow deferrable timers without the TIMER_PINNED flag to run
on any active CPU.

[rishabhb@codeaurora.org: Resolved conflicts in kernel/time/timer.c due to
upstream commit hrtimer: Optimize the hrtimer code by using static keys for
 migration_enable/nohz_active]
Change-Id: I8e6b77cef972589912ad18f324c46c936fbbb96f
Signed-off-by: Kyle Yan <kyan@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>