pci_host_generic: Include the bridge's device name in rman descriptions

The rman description strings now match those used in the PCI-PCI
bridge driver. Using more specific names removes ambiguity in devinfo -u
output on systems with multiple host to PCI bridges.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43890

(cherry picked from commit 51f8ac224f3b87555ea17c7e7c4015a2a2b8b191)