Migrate the PTE format for book-e to standardize on the 'indirect PTE' format

Summary:
The revised Book-E spec, adding the specification for the MMUv2 and e6500,
includes a hardware PTE layout for indirect page tables. In order to support
this in the future, migrate the PTE format to match the MMUv2 hardware PTE
format.

Test Plan: Boot tested on a P5020 board. Booted to multiuser mode.

Differential Revision: https://reviews.freebsd.org/D5224