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 <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>