drivers: cpuidle: lpm-levels: Log hotplug events

Add debug events for CPU_STARTING and CPU_DYING events when a CPU is
hotplugged. The events will help correlate with the TZ events using the
ARM timers counts for debug.

Change-Id: Ieb6449575b4a95974b787ceea430fc5362c77c29
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>