Refactor: rename namespace witherspoon to phosphor

This repo now becomes phosphor-power, rename namespace witherspoon to
phosphor to make it generic.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: Icb9f8a7c33065add99f2bf42ed55745a28fa0424
diff --git a/device_monitor.hpp b/device_monitor.hpp
index 2d55af3..7960d1c 100644
--- a/device_monitor.hpp
+++ b/device_monitor.hpp
@@ -8,7 +8,7 @@
 #include <sdeventplus/event.hpp>
 #include <sdeventplus/utility/timer.hpp>
 
-namespace witherspoon
+namespace phosphor
 {
 namespace power
 {
@@ -79,4 +79,4 @@
 };
 
 } // namespace power
-} // namespace witherspoon
+} // namespace phosphor