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/CP15.cpp

@@ -668,7 +668,7 @@ void ARMv5::CP15Write(u32 id, u32 val)
         Log(LogLevel::Debug, "unknown CP15 write op %03X %08X\n", id, val);
 }
 
-u32 ARMv5::CP15Read(u32 id)
+u32 ARMv5::CP15Read(u32 id) const
 {
     //printf("CP15 read op %03X %08X\n", id, NDS::ARM9->R[15]);