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/SPU.h

@@ -237,7 +237,7 @@ public:
     void TrimOutput();
     void DrainOutput();
     void InitOutput();
-    int GetOutputSize();
+    int GetOutputSize() const;
     void Sync(bool wait);
     int ReadOutput(s16* data, int samples);
     void TransferOutput();