SI: Namespace device classes

Places all of the SI code under the SerialInterface namespace instead of
only the main source file. This keeps all SI code under a common name,
as well as out of the global namespace

Source/Core/Core/ConfigManager.h

@@ -252,7 +252,7 @@ struct SConfig : NonCopyable
   std::string m_strGbaCartA;
   std::string m_strGbaCartB;
   TEXIDevices m_EXIDevice[3];
-  SIDevices m_SIDevice[4];
+  SerialInterface::SIDevices m_SIDevice[4];
   std::string m_bba_mac;
 
   // interface language