PM / domains: Keep declaration of dev_power_governors together

This is a pure code cleanup in the header file for the PM domain. No
functional change.

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