This is largely a straight-forward cleave of the 32-bit and 64-bit page
table specifics, along with the mmu_booke_*() functions that are wholely
different between the two implementations.

The ultimate goal of this is to make it easier to reason about and
update a specific implementation without wading through the other
implementation details. This is in support of changing the page table
format for 64-bit to a 4-level full hierarchical page table, instead of
the current hashed top level with holes.