powerpc/booke: Change Book-E 64-bit pmap to 4-level table

Summary:
The existing page table is fraught with errors, since it creates a hole
in the address space bits. Fix this by taking a cue from the POWER9
radix pmap, and make the page table 4 levels, 52 bits.

Reviewed by: bdragon
Differential Revision: https://reviews.freebsd.org/D24220