ARM: imx6: add pci config space as platform resource

Fixes "imx6q-pcie 1ffc000.pcie: missing *config* reg space"
error exposed by new versions of the designware pcie driver.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>