libpci: Define new string property PCI_FILL_DRIVER

libpci: Define new string property PCI_FILL_DRIVER

This change extends libpci library and allows providers to fill
PCI_FILL_DRIVER via native system APIs. As it is string property there is
no need to increase ABI version.

Intended usage in application is just:

  const char *driver = pci_get_string_property(d->dev, PCI_FILL_DRIVER);