pci: Set the domain in the pci_generic driver

Tell bus_dma(9) which NUMA domain the PCI driver is closest to so it
can allocate memory from there when possible.

Reviewed by: markj, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42186

(cherry picked from commit 7098f3c7569c21ecdfc990772e96a416f1a94eee)