soc: qcom: msm_perf: Add sysfs node to report cpu hotplug events

cpufreq governor tunables could be updated
only when cpu is online. Userspace might need
to apply and restore these tunables depending
on the power/perf need.
Provide sysfs node for userspace to poll over
and which gets notified on CPU_ONLINE and CPU_DEAD
events for any cpu. This mechanism could be
utilized to know when cpus come online
and then restoring or applying cpufreq governor
tunables. CPU_DEAD notification is also provided
for completeness.

Change-Id: I221a97c49a72da217e5e4b46a16a26042ef5c83f
Signed-off-by: Omprakash Dhyade <odhyade@codeaurora.org>