riscv: Implement pmap_mapdev_attr

[jrtc27]

Summary

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.

MFC after: 1 week

[markj]

sys/riscv/riscv/pmap.c

919

[mhorne]

sys/riscv/include/pmap.h

148

It would be nice to eventually land on a common interface for pmap_kenter() across various architectures, but matching arm64 here is sensible and required.

[jrtc27]

sys/riscv/include/pmap.h

148

Yeah, though most other architectures assume size is PAGE_SIZE so would require a bit more work than just adding arguments to their implementations

sys/riscv/riscv/pmap.c

919

Good point, will do on commit