[CORE] timer: Protect active event list on each host CPU

Timer events when started are always scheduled on current host CPU.

In case of SMP systems, the timer events may be stopped on-the-fly
from any other host CPU hence, we need to protect the active event
list on each host CPU.

This patch adds protection to active event list on each host CPU.

Signed-off-by: Anup Patel <anup@brainfault.org>