Introduce 64-bit PowerPC Book-E support

Extend the Book-E pmap to support 64-bit operation. Much of this was taken from
Juniper's Junos FreeBSD port. It uses a 3-level page table (page directory
list -- PP2D, page directory, page table), but has gaps in the page directory
list where regions will repeat, due to the design of the PP2D hash (a 20-bit gap
between the two parts of the index). In practice this may not be a problem
given the expanded address space. However, an alternative to this would be to
use a 4-level page table, like Linux, and possibly reduce the available address
space; Linux appears to use a 46-bit address space. Alternatively, a cache of
page directory pointers could be used to keep the overall design as-is, but
remove the gaps in the address space.

This includes a new kernel config for 64-bit QorIQ SoCs, based on MPC85XX, with
the following notes:

• The DPAA driver has not yet been ported to 64-bit so is not included in the kernel config.
• This has been tested on the AmigaOne X5000, using a MD_ROOT compiled in (total size kernel+mdroot must be under 64MB).
• This can run both 32-bit and 64-bit processes, and has even been tested to run a 32-bit init with 64-bit children.

Many thanks to stevek and marcel for getting Juniper's FreeBSD patches open
sourced to be used here, and to stevek for reviewing, and providing some
historical contexts on quirks of the code.

Reviewed by: stevek
Obtained from: Juniper (in part)
MFC after: 2 months
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D9433