Fix STPC CPU identification

Let port 22h/23h registers >= 0xc0 fall through to the Cyrix port 22h/23h code in cpu.c

src/chipset/stpc.c

@@ -492,6 +492,8 @@ stpc_reg_read(uint16_t addr, void *priv)
 
     if (addr == 0x22)
 	ret = dev->reg_offset;
+    else if (dev->reg_offset >= 0xc0)
+    	return 0xff; /* Cyrix CPU registers: let the CPU code handle those */
     else
 	ret = dev->regs[dev->reg_offset];