PM / Domains: Add power-on function using names to identify domains

It sometimes is necessary to turn on a given PM domain when only
the name of it is known and the domain pointer is not readily
available.  For this reason, add a new helper function,
pm_genpd_name_poweron(), allowing the caller to turn on a PM domain
using its name for identification.  To avoid code duplication,
move the domain lookup code to a separate function.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
diff --git a/include/linux/pm_domain.h b/include/linux/pm_domain.h
index 8dbf48b..d9d6083 100644
--- a/include/linux/pm_domain.h
+++ b/include/linux/pm_domain.h
@@ -161,6 +161,7 @@
 			  struct dev_power_governor *gov, bool is_off);
 
 extern int pm_genpd_poweron(struct generic_pm_domain *genpd);
+extern int pm_genpd_name_poweron(const char *domain_name);
 
 extern bool default_stop_ok(struct device *dev);
 
@@ -240,6 +241,10 @@
 {
 	return -ENOSYS;
 }
+static inline int pm_genpd_name_poweron(const char *domain_name)
+{
+	return -ENOSYS;
+}
 static inline bool default_stop_ok(struct device *dev)
 {
 	return false;