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.cpp

@@ -235,7 +235,7 @@ void DSi_AES::ProcessBlock_CTR()
 }
 
 
-u32 DSi_AES::ReadCnt()
+u32 DSi_AES::ReadCnt() const
 {
     u32 ret = Cnt;