pci_host_generic*: basic implementation of bus range

Both ACPI and FDT support bus ranges for pci host bridges. Update
pci_host_generic*.[ch] with a default implementation to support this.
This will be used in the next set of changes for ACPI based host
bridge. No functional changes in this commit.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D17657