powerpc psim: Use bus_generic_rman_*

• Split out bits of iobus_activate/deactive_resource to implement iobus_map/unmap_resource.

• Use bus_generic_rman_* methods in several bus methods to handle memory and I/O port resources.

• Add an implementation of bus_adjust_resource.

Reviewed by: imp, jhibbits
Differential Revision: https://reviews.freebsd.org/D43433