fix: mvebu: pcie_dw: Allow probing empty PCIe slots

fix: mvebu: pcie_dw: Allow probing empty PCIe slots

This patch allows probing all PCIe nodes defined in DTS
even if there no device connected to such node (no link).
Without this fix the driver returns -ENODEV when the PCIe
link is down. As result the pci_init function stops
scanning bus on first empty PCIe slot and all devices
located in higher numbered buses are not discovered.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Igal Liberman <igall@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>