While reading code I noticed we need to call thread_info_init before
mem_init to be correct. In practice it hasn't mattered because cpu0
is calling mem_init, and it's cpu index is 0, which is what a fresh
first boot would have it be anyway. With reset tests (coming soon)
combined with mmu tests (hopefully coming someday) it could be
problematic though, so let's fix it. Additionally comment the order
to make sure we maintain it. The io_init dependency on mem_init is
because io_init calls init functions that make use of heap allocation.

Signed-off-by: Andrew Jones <drjones@redhat.com>