pci_host_generic_fdt.c: Add support for mapping dts nodes to PCI devices

Some arm64 SoCs have nodes in their fdts that describe devices
connected to the internal PCI bus. One such SoC is Freescale LS1028A.
It expects the nodes to be mapped to devices enumerated using the standard
PCI method. Mapping is done by reading device and function ids from "reg"
property. Information is dts is used to describe MDIO/PHY connected
to a given interface.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30180