cpu/hotplug: Add cpuhp_latency trace event

Add ftrace event trace_cpuhp_latency to track cpu
hotplug latency and this entry is useful in power,
performance and debug analysis.

Change-Id: Ie6b2a91027c665ccaf6325b0836bb629a4e91dd7
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>