PM: domains: Make of_genpd_add_subdomain() return -EPROBE_DEFER

Driver of a power domain provider may not be ready at the time of
of_genpd_add_subdomain() invocation. Make this function to return
-EPROBE_DEFER instead of -ENOENT in order to remove a need from
power domain drivers to handle the error code specially.
Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Nicolas Chauvet <kwizart@gmail.com>
Tested-by: Matt Merhar <mattmerhar@protonmail.com>
[tested on NVIDIA Tegra20/30/124 SoCs]
Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/domain.c

@@ -2496,7 +2496,7 @@ int of_genpd_add_subdomain(struct of_phandle_args *parent_spec,
 out:
 	mutex_unlock(&gpd_list_lock);
 
-	return ret;
+	return ret == -ENOENT ? -EPROBE_DEFER : ret;
 }
 EXPORT_SYMBOL_GPL(of_genpd_add_subdomain);