Starting point for the new MMIO interface

Design doc:
    https://docs.google.com/document/d/11qcGCWLne1wYvmtFaSrOKZt_vHxXrcWcZsdWJ-MJnyo/edit

The code is currently not used. Migration plan:
    1. Implement MMIO access via MMIO::Mapping in parallel to the current
       method.
    2. Implement all existing MMIO handlers via the new interface.
    3. Remove the old hwRead/hwReadWii/hwReadIOBridge code.
    4. Implement JIT optimizations for MMIO accesses.

Source/Core/Core/HW/Memmap.h

@@ -28,6 +28,7 @@
 
 // Global declarations
 class PointerWrap;
+namespace MMIO { class Mapping; }
 
 typedef void (*writeFn8 )(const u8, const u32);
 typedef void (*writeFn16)(const u16,const u32);
@@ -83,6 +84,9 @@ enum
 #endif
 };
 
+// MMIO mapping object.
+extern MMIO::Mapping* mmio_mapping;
+
 // Init and Shutdown
 bool IsInitialized();
 void Init();