soc: qcom: msm_perf: Rename the hotplug structure as cluster

The structure cpu_hp responsible for keeping stats related to CPU
hotplug operations is basically doing so for each cluster. Renaming
it as 'cluster' in this patch so that it can be used to store any
cluster-specific stats in future modifications to the code.

Change-Id: I8ae34c883b932631341379baaa6c39e144c5c86f
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>