lpm: Add sysfs node to display total sleep time

Add sysfs node to query the total sleep time of a cpu since debugfs node
is not exposed to the userspace service.

Command:
cat /sys/module/lpm_stats/cpu0/total_sleep_time_secs
cat /sys/module/lpm_stats/cpu1/total_sleep_time_secs
cat /sys/module/lpm_stats/cpu2/total_sleep_time_secs
cat /sys/module/lpm_stats/cpu3/total_sleep_time_secs

CRs-fixed: 935207
Change-Id: I45be0a8be29932816aa42e097657a2a60933b986
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>