amd64: for LA57, connect DMAP directly to PML5, currently reserving 32 slots

This removes the 4T limit on the amd64 usable phys memory.

Other large KVA consumers like large map, should be moved out of the
top PML4 entry as well. After that, kmem on LA57 amd64 can be extended.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D51085