riscv: Rename struct pmap's pm_l1 field to pm_top

In SV48 mode, the top-level page will be an L0 page rather than an L1
page. Rename the field accordingly. No functional change intended.

Reviewed by: alc, jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34270