Bus: Add stub for SIO2 accesses

But only enable it when using a PS2 BIOS. I could put the check in the
handler registration, but realistically this is basically never going to
be called, so better to keep things simple.

src/core/bus.h

@@ -67,7 +67,10 @@ enum : u32
   MDEC_MASK = MDEC_SIZE - 1,
   SPU_BASE = 0x1F801C00,
   SPU_SIZE = 0x400,
-  SPU_MASK = 0x3FF,
+  SPU_MASK = SPU_SIZE - 1,
+  SIO2_BASE = 0x1F808000,
+  SIO2_SIZE = 0x1000,
+  SIO2_MASK = SIO2_SIZE - 1,
   EXP2_BASE = 0x1F802000,
   EXP2_SIZE = 0x2000,
   EXP2_MASK = EXP2_SIZE - 1,