PM / Runtime: Implement the pm_generic_runtime functions for CONFIG_PM

The pm_generic_runtime_suspend|resume functions were implemented within
CONFIG_PM_RUNTIME.

As we also may use runtime PM callbacks during system suspend, to put
devices into low power state, we need to move the implementation of
pm_generic_runtime_suspend|resume to CONFIG_PM.

This change gives a power domain provision to invoke a platform
driver's runtime PM callback from a power domain's system PM callback.
This were earlier prevented by the platform bus, since it uses the
pm_generic_runtime_suspend|resume functions as runtime PM callbacks.

Cc: Kevin Hilman <khilman@linaro.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>