of: unittest: Add pci_dt_testdrv pci driver

pci_dt_testdrv is bound to QEMU PCI Test Device. It reads
overlay_pci_node fdt fragment and apply it to Test Device. Then it
calls of_platform_default_populate() to populate the platform
devices.

Tested-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://lore.kernel.org/r/1692120000-46900-6-git-send-email-lizhi.hou@amd.com


Signed-off-by: Rob Herring <robh@kernel.org>