WII_IPC: fix homebrew that uses the disc drive

Dolphin never emulated the AHBPROT register before, but the default
value when reading from unimplemented MMIO registers used to be -1,
which happened to match what AHBPROT reads as when all restrictions are
disabled. In 6f25e20c6a I changed the
default to 0 to match observed hardware behavior in the memory range of
the command processor. This broke libogc's DI_Init() which checks
AHBPROT for full hardware access (presumably to ensure that bypassing
IPC for Video DVDs will work).

Source/Core/Core/HW/WII_IPC.cpp

@@ -41,6 +41,8 @@ enum
   ARM_IRQFLAG = 0x38,
   ARM_IRQMASK = 0x3c,
 
+  AHBPROT = 0x64,
+
   GPIOB_OUT = 0xc0,
   GPIOB_DIR = 0xc4,
   GPIOB_IN = 0xc8,
@@ -169,6 +171,9 @@ void WiiIPC::RegisterMMIO(MMIO::Mapping* mmio, u32 base)
                                                         0);
                  }));
 
+  // Dolphin currently does not emulate any hardware access restrictions.
+  mmio->Register(base | AHBPROT, MMIO::Constant<u32>(0xFFFFFFFF), MMIO::InvalidWrite<u32>());
+
   mmio->Register(base | GPIOB_OUT, MMIO::DirectRead<u32>(&m_gpio_out.m_hex),
                  MMIO::ComplexWrite<u32>([](Core::System& system, u32, u32 val) {
                    auto& wii_ipc = system.GetWiiIPC();