dt-bindings: PCI: designware: Remove the num-lanes from Required properties

The num-lanes is not a mandatory property, e.g. on FSL
Layerscape SoCs, the PCIe link training is completed
automatically based on the selected SerDes protocol, it
does not need the num-lanes to set-up the link width.

Currently it is both a Required and Optional property,
let's remove it from the Required properties.

Signed-off-by: Hou Zhiqiang <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Reviewed-by: Andrew Murray <[e-mail]>

Documentation/devicetree/bindings/pci/designware-pcie.txt

···
 	     the ATU address space.
     (The old way of getting the configuration address space from "ranges"
     is deprecated and should be avoided.)
-- num-lanes: number of lanes to use
 RC mode:
 - #address-cells: set to <3>
 - #size-cells: set to <2>