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/ARM.h

@@ -80,7 +80,7 @@ public:
     virtual void ExecuteJIT() = 0;
 #endif
 
-    bool CheckCondition(u32 code)
+    bool CheckCondition(u32 code) const
     {
         if (code == 0xE) return true;
         if (ConditionTable[code] & (1 << (CPSR>>28))) return true;
@@ -109,7 +109,7 @@ public:
         if (v) CPSR |= 0x10000000;
     }
 
-    inline bool ModeIs(u32 mode)
+    inline bool ModeIs(u32 mode) const
     {
         u32 cm = CPSR & 0x1f;
         mode &= 0x1f;
@@ -315,7 +315,7 @@ public:
     void ICacheInvalidateAll();
 
     void CP15Write(u32 id, u32 val);
-    u32 CP15Read(u32 id);
+    u32 CP15Read(u32 id) const;
 
     u32 CP15Control;