riscv: adjust physmem reservation
Make sure we do this BEFORE pmap_bootstrap().
Reviewed by: markj, jhb
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45325
(cherry picked from commit 0e4e77072f65df71644140e41229b868a73f3371)