In the unit test I'm adding in the next commit, I want to call
MemoryManager::Init, but initializing all the hardware that
MemoryManager::InitMMIO calls into would be cumbersome.
Calling MemoryManager::InitMMIO from MemoryManager::Init was a bit
strange anyway. Because MemoryManager::Init is called about halfway
through HW::Init, some of the hardware that MemoryManager::InitMMIO
calls into isn't initialized yet.