BACKPORT: PM/sleep: Expose suspend stats in sysfs

Userspace can get suspend stats from the suspend stats debugfs node.
Since debugfs doesn't have stable ABI, expose suspend stats in
sysfs under /sys/power/suspend_stats.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 2c8db5bef9fb442a5a0d2fee28aed20100362ce3)
[ fixed conflicts in Documentation ]
Bug: 129087298
Change-Id: Ia8f0d8551ae693cb5f3e1abc609fb9bbfeb695f5
Signed-off-by: Tri Vo <trong@google.com>