imx: ventana: default pci to disabled

imx: ventana: default pci to disabled

The IMX6 PCIe host controller does not have a proper reset and as such there
are several issues that can arise if PCI is enabled in the bootloader follwed
by Linux trying to re-configure LTSSM and/or toggling PERST# to the devices.

For now, the best approach seems to default to disabling PCI by defaulting
pciedisable=1. This can be overridden by the user if they need PCI in the
bootloader, for example:
 - GW552x needing ethernet access in bootloader
 - GW16082 expansion board needing a device-tree fixup for irq mapping

Signed-off-by: Tim Harvey <tharvey@gateworks.com>