[PowerPC] Fix Book-E direct map for >=16G ram on e5500

It turns out the maximum TLB1 page size on e5500 is 4G, despite the format
being defined for up to 1TB.

So, we need to clamp the DMAP TLB1 entries to not attempt to create 16G or
larger entries.

Fixes boot on my X5000 in which I just installed 16G of RAM.

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D23244