pci_host_generic: Don't rewrite resource start address for translation

Allocate resources from the parent device for decoded physical address
ranges. When child resources suballocated from rman's are mapped,
translate those mapping requests into a mapping request of the
associated physical address range in a bus_map_resource method.

While here, convert generic_pcie_rman to a bus_get_rman method and use
bus_generic_rman_* for operations on child resources.

Factor out a generic_pcie_containing_range to share logic between
bus_translate_resource and bus_*map_resource.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43894