PCI: imx6: Allow asynchronous probing

Establishing a PCIe link can take a while; allow asynchronous probing so
that link establishment can happen in the background while other devices
are being probed.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>