newbus: Add a set of bus resource helpers for nexus-like devices

These routines can be used to implement
bus_alloc/adjust/activate/deactive/release_resource on bus drivers
which suballocate resources from rman(9) resource managers.

These methods require a new bus_get_rman method in the bus driver to
return the suitable rman for a given resource type. The
activate/deactivate helpers also require the bus to implement the
bus_map/ummap_resource methods.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D42739