PM / Domains: Fix missing default_power_down_ok comment

Commit fc5cbf0c (PM / Domains: Support for multiple states) split
out some code out of default_power_down_ok() function so the
documentation has to be moved to appropriate place.

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