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

[jhibbits]

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 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.

[bdragon]

I definitely approve conceptually.

Haven't had a chance to properly test this on the x5000 and rb800 yet, but will try to get to it.

[bdragon]

Compiles and boots on RB800 and X5000, and is a lot easier to reason about than the combined version.

[bdragon]

sys/powerpc/booke/pmap_64.c

39 ↗

(On Diff #69250)

Should verify and move these notes to pmap_32.c. Can be done in a later revision just as easily though.