DiscIO: Add GetRegion function and Region enum

Instead of needing different switch cases for
converting countries to regions in multiple places,
we now only need a single country-to-region switch case
(in DiscIO/Enums.cpp), and we get a nice Region type.

Source/Core/Core/Boot/Boot.cpp

@@ -419,7 +419,7 @@ bool CBoot::BootUp()
 
     // Poor man's bootup
     if (_StartupPara.bWii)
-      SetupWiiMemory(DiscIO::Country::COUNTRY_UNKNOWN);
+      SetupWiiMemory(DiscIO::Region::UNKNOWN_REGION);
     else
       EmulatedBS2_GC(true);