riscv: fix L0 PTE setup (Sv48)

Per the Privilege Spec, the Accessed (A) or Dirty (D) bits must only be
set for a leaf PTE.

It seems newer versions of QEMU have started to enforce this
requirement, and without this change, pmap_bootstrap() hangs when
switching to Sv48 mode.

Reviewed by: jrtc27, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45210

(cherry picked from commit d7adf3b47a058552270fb2603cc888f40eba3e17)