Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/DSi_AES.h

@@ -54,7 +54,7 @@ public:
     void Reset();
     void DoSavestate(Savestate* file);
 
-    u32 ReadCnt();
+    u32 ReadCnt() const;
     void WriteCnt(u32 val);
     void WriteBlkCnt(u32 val);