powerpc ps3bus: Use bus_generic_rman_*

• Split out part of ps3bus_activate_resource into a ps3bus_map_resource method.

• Implement ps3bus_unmap_resource via pmap_unmapdev.

• Use bus_generic_rman_* to add bus_adjust_resource, bus_deactivate_resource, and bus_release_resource methods.

Reviewed by: imp, jhibbits
Differential Revision: https://reviews.freebsd.org/D43432