riscv: Implement missing nexus methods

This is required for the SiFive FU740's PCIe controller. Copied from
arm64 with the only difference being changing pmap_mapdev_attr to
pmap_mapdev as riscv only has the latter.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31032