ARM: dts: imx6sx: Fix the PCI representation

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>