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.cpp

@@ -956,7 +956,7 @@ void SPU::InitOutput()
     Platform::Mutex_Unlock(AudioLock);
 }
 
-int SPU::GetOutputSize()
+int SPU::GetOutputSize() const
 {
     Platform::Mutex_Lock(AudioLock);