pci_host_generic: allocate resources against devices

Fix up pci_host_generic.c and pci_host_generic_fdt.c to allocate
resources against devices that requested them. Currently the
allocation happens against the pcib, which is incorrect.

This is needed for the upcoming changes for fixing up
pci_host_generic_acpi.c

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D17656