PM / core: Clean up some function headers in power.h

The power.h is a bit messy due to the various existing CONFIG_PM* Kconfig
combinations. However the final section for wakeup_source_sysfs*() can be
moved inside one of the existing sections rather than adding yet another
one, so let's do that to clean up the code a little bit.

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