arm: imx6: fix pcie by disabling rc/ep test mode

Fix PCIe operation by disabling RC/EP test mode which according to the
following community forum post is only used on the PCIe EP/RC
validation system and otherwise prevent PCIe from working properly:

https://community.freescale.com/thread/328257

Conflicts:
	arch/arm/configs/apalis_imx6_defconfig
        (doesn't exist in Boundary Devices tree)