LinuxKPI: Implement _ioremap_attr for riscv

Now that riscv implements pmap_mapdev_attr we can enable the non-stub
implementation for riscv, which is needed for drm-kmod to not fail at
run time for drivers that need to map I/O regions.

Reviewed by: hselasky, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32446