arm64 thunder: Use bus_generic_rman_* in the pcie_pem driver

• Implement bus_map/unmap_resource pulling bits from the previous thunder_pem_activate/deactivate_resource.

• Use bus_generic_rman_* in various routines for memory and I/O port resources.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43439