powerpc/booke: Split out 32- and 64- bit pmap details from main body

Summary:
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 further changes to the 64-bit
pmap.

Reviewed by: bdragon
Differential Revision: https://reviews.freebsd.org/D23983