devfreq: memlat: Move hp_restart_events within the HOTPLUG ifdef

To compile for targets where HOTPLUG is disabled move the
msm_hp_restart_events function within the HOTPLUG ifdef code.

Change-Id: If316b76812dd8e3f45a73fe1d4f908b5ffabeef4
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>