Use C++17 format for nested namespace definitions

Use the new C++17 format for defining nested namespaces in the
phosphor-regulators application.

The definition
  namespace A::B::C
  {
  ...
  }

is now equivalent to
   namespace A
   {
   namespace B
   {
   namespace C
   {
   ...
   }
   }
   }

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I284055bc94578a45d04d3561f511c0e347a8ecef
diff --git a/phosphor-regulators/src/actions/action_utils.hpp b/phosphor-regulators/src/actions/action_utils.hpp
index 9ddff4f..2cf9c7d 100644
--- a/phosphor-regulators/src/actions/action_utils.hpp
+++ b/phosphor-regulators/src/actions/action_utils.hpp
@@ -21,13 +21,7 @@
 #include <memory>
 #include <vector>
 
-namespace phosphor
-{
-namespace power
-{
-namespace regulators
-{
-namespace action_utils
+namespace phosphor::power::regulators::action_utils
 {
 
 /**
@@ -58,7 +52,4 @@
     return returnValue;
 }
 
-} // namespace action_utils
-} // namespace regulators
-} // namespace power
-} // namespace phosphor
+} // namespace phosphor::power::regulators::action_utils