RISC-V: copy the DTB to early KVA

The location of the device-tree blob is passed to the kernel by the
previous booting stage (i.e. BBL or OpenSBI). Currently, we leave it
untouched and mark the 1MB of memory holding it as unavailable.

Instead, do what is done by other fake_preload_metadata() routines and
copy to the DTB to KVA space. This is more in line with what loader(8)
will provide us in the future, and it allows us to reclaim the hole in
physical memory.

Reviewed by: markj, kp (earlier version)
Differential Revision: https://reviews.freebsd.org/D24152