arm nexus: Use bus_generic_rman_*

• Implement bus_get_rman pulling bits from nexus_alloc_resource.

• Implement bus_map/unmap_resource pulling bits from nexus_activate/deactivate_resource.

• Use bus_generic_rman_* for bus_alloc/adjust/activate/deactivate/release_resource except for custom interrupt activate/deactivate logic still in nexus_activate/deactivate_resource.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43437