powerpc: Merge all pmap struct definitions

Summary:
A few ports fail to build due to missing pmap-related definitions, which are
specific per-pmap type. This tries to appease those ports, by merging all
pmaps together.

A future change will move the inline page directory out of the Book-E pmap,
to eliminate the last #ifdefs in pmap.h and complete the merge.

Reviewed By: luporl
Differential Revision: https://reviews.freebsd.org/D20119