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/gpio.hpp b/gpio.hpp
index 2742d4f..edb9b77 100644
--- a/gpio.hpp
+++ b/gpio.hpp
@@ -7,7 +7,7 @@
 #include <string>
 #include <type_traits>
 
-namespace witherspoon
+namespace phosphor
 {
 namespace gpio
 {
@@ -114,4 +114,4 @@
 };
 
 } // namespace gpio
-} // namespace witherspoon
+} // namespace phosphor