msm: pm: add new stat variable for reporting suspend stats

Incorrectly reporting suspend stats because of moving timekeeping
logic to prepare/resume callbacks in suspend. Declare a new
global system_suspend stat variable for reporting suspend
stats and remove the suspend stats for each core.

CRs-fixed: 584850
Change-Id: I1ad3dbf7fe6f9a3bf56f78706d3086d42f41ce13
Signed-off-by: Anil Kumar Mamidala <amami@codeaurora.org>