arm: mvebu: turris_mox: temporary fix for PCIe cards to work

For some reason the pci-aardvark driver in Linux always reports
	advk-pcie d0070000.pcie: link never came up
if the PCIe controller is probed in U-Boot.

Temporarily disable PCIe probing in U-Boot until this issue is solved in
the pci-aardvark driver in U-Boot.

Signed-off-by: Marek Behún <marek.behun@nic.cz>