Restructure device mappings for Book-E.

Summary:
There is currently a 1GB hole between user and kernel address spaces
into which direct (1:1 PA:VA) device mappings go. This appears to go largely
unused, leaving all devices to contend with the 128MB block at the end of the
32-bit space (0xf8000000-0xffffffff). This easily fills up, and needs to be
densely packed. However, dense packing wastes precious TLB1 space, of which
there are only 16 (e500v2) or 64(e5500) entries available.

Change this by using the 1GB space for all device mappings, and allow the kernel
to use the entire upper 1GB for KVA. This also allows us to use sparse device
mappings, freeing up TLB entries.

Test Plan: Boot tested on p5020.

Differential Revision: https://reviews.freebsd.org/D5832