PM: sleep: stats: Define suspend_stats next to the code using it

It is not necessary to define struct suspend_stats in a header file and the
suspend_stats variable in the core device system-wide PM code.  They both
can be defined in kernel/power/main.c, next to the sysfs and debugfs code
accessing suspend_stats, which can be static.

Modify the code in question in accordance with the above observation and
replace the static inline functions manipulating suspend_stats with
regular ones defined in kernel/power/main.c.

While at it, move the enum suspend_stat_step to the end of suspend.h which
is a more suitable place for it.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>