msm: rq_stats: update related_cpus in the cpu_load runtime

"related_cpu" indicates the list of CPUs that need some
sort of frequency coordination, whether software or hardware.
ex:- if all cpus shares the same clock, all 4 cpus belongs to
related_cpus.

cpufreq driver updates the related_cpus during hotplug offline/online
events. Hence update related_cpus for all cpus during hotplug notifier.

CRs-fixed: 548631
Change-Id: I6543ddaeebf73d18334066b090ee7c4a72466e4a
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>