Currently the imx pci driver fails to probe on i.MX6SX.

Fix the PCI representation so that the driver can be probed and
PCI devices can be detected.

Tested on a im6sx-sdb board:

00:00.0 Class 0604: 16c3:abcd
01:00.0 Class 0280: 8086:08b1
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>