Add WAITCNT but do nothing for now

gba_memory.c

@@ -1346,6 +1346,7 @@ cpu_alert_type function_cc write_io_register16(u32 address, u32 value)
 
     // WAITCNT
     case 0x204:
+      write_ioreg(REG_WAITCNT, value);
       break;
 
     // Halt

gba_memory.h

@@ -150,6 +150,7 @@ typedef enum
   REG_RCNT = 0x9A,
   REG_IE = 0x100,
   REG_IF = 0x101,
+  REG_WAITCNT = 0x102,
   REG_IME = 0x104,
   REG_HALTCNT = 0x180
 } hardware_register;