powerpc powermac: Use bus_generic_rman_*

Implement bus_map/unmap_resource and add bus_get_rman for use by
bus_generic_rman_*.

Reviewed by: imp, jhibbits
Differential Revision: https://reviews.freebsd.org/D43435

(cherry picked from commit af081ec6f7cf4b764a1b417135440cf05cf1a174)