Make bus_space_generic properly map/unmap memory (using pmap_mapdev and
pmap_unmapdev respectively) so that resources are properly managed.

This is work originally done by kan@. Stanislav picked it up as part
of his Mediatek SoC work.

Tested:

• Carambola2, AR933x SoC

Submitted by: Stanislav Galabov <sgalabov@gmail.com>
Reviewed by: kan
Differential Revision: https://reviews.freebsd.org/D5184