Enhance book-e pmap for 36-bit physaddr

Summary:
This is (probably step 1) of enhancing the book-e pmap to support the full
36-bit physical address space on Freescale e500 and e5500 cores.

Thus far it has only been regression tested on one platform. Since I only have
one other Book-E platform (e5500), that needs work beyond this, I haven't yet
tested it on this.

Test Plan: Regression tested on my RouterBoard RB800.

Reviewed By: marcel
Differential Revision: https://reviews.freebsd.org/D3027