riscv: Implement pmap_mapdev_attr

This is needed for LinuxKPI's _ioremap_attr. This reuses the generic
implementation introduced for aarch64, and itself requires implementing
pmap_kenter, which is trivial to do given riscv currently treats all
mapping attributes the same due to the Svpbmt extension not yet being
ratified and in hardware.

Reviewed by: markj, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32445