PCI: layerscape: Convert to builtin_platform_driver()

fw_devlink will defer the probe until all suppliers are ready. We can't
use builtin_platform_driver_probe() because it doesn't retry after probe
deferral. Convert it to builtin_platform_driver().

Link: https://lore.kernel.org/r/20210120105246.23218-1-michael@walle.cc


Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>