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_SPI_TSC.cpp

@@ -121,7 +121,7 @@ void DSi_TSC::SetTouchCoords(u16 x, u16 y)
     }
 }
 
-void DSi_TSC::MicInputFrame(s16* data, int samples)
+void DSi_TSC::MicInputFrame(const s16* data, int samples)
 {
     if (TSCMode == 0x00) return TSC::MicInputFrame(data, samples);