HW: Use unsigned indices in RegisterMMIO where applicable

base is an unsigned variable, so we can make things little more
consistent by making the loop index unsigned so we aren't doing bit
arithmetic with signed types.

MemoryInterface already does this, so we can leave it alone.

No behavioral changes, just a consistency thing.

Source/Core/Core/HW/DSP.cpp

@@ -453,7 +453,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
                  MMIO::InvalidWrite<u16>());
 
   // 32 bit reads/writes are a combination of two 16 bit accesses.
-  for (int i = 0; i < 0x1000; i += 4)
+  for (u32 i = 0; i < 0x1000; i += 4)
   {
     mmio->Register(base | i, MMIO::ReadToSmaller<u32>(mmio, base | i, base | (i + 2)),
                    MMIO::WriteToSmaller<u32>(mmio, base | i, base | (i + 2)));

Source/Core/Core/HW/ProcessorInterface.cpp

@@ -142,7 +142,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
                  MMIO::InvalidWrite<u32>());
 
   // 16 bit reads are based on 32 bit reads.
-  for (int i = 0; i < 0x1000; i += 4)
+  for (u32 i = 0; i < 0x1000; i += 4)
   {
     mmio->Register(base | i, MMIO::ReadToLarger<u16>(mmio, base | i, 16),
                    MMIO::InvalidWrite<u16>());

Source/Core/Core/HW/VideoInterface.cpp

@@ -433,7 +433,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
                  }));
 
   // Map 8 bit reads (not writes) to 16 bit reads.
-  for (int i = 0; i < 0x1000; i += 2)
+  for (u32 i = 0; i < 0x1000; i += 2)
   {
     mmio->Register(base | i, MMIO::ReadToLarger<u8>(mmio, base | i, 8), MMIO::InvalidWrite<u8>());
     mmio->Register(base | (i + 1), MMIO::ReadToLarger<u8>(mmio, base | i, 0),
@@ -441,7 +441,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
   }
 
   // Map 32 bit reads and writes to 16 bit reads and writes.
-  for (int i = 0; i < 0x1000; i += 4)
+  for (u32 i = 0; i < 0x1000; i += 4)
   {
     mmio->Register(base | i, MMIO::ReadToSmaller<u32>(mmio, base | i, base | (i + 2)),
                    MMIO::WriteToSmaller<u32>(mmio, base | i, base | (i + 2)));