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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 82f4e0d0f06456f2d070d2e0692a8f318f4eb90b)