dt-bindings: PCI: dra7xx: Add properties to enable x2 lane in dra7

Add syscon properties required for configuring PCIe in x2 lane mode.

Signed-off-by: Kishon Vijay Abraham I <[e-mail]>
Signed-off-by: Sekhar Nori <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/pci/ti-pci.txt

···
  - ti,hwmods : Name of the hwmod associated to the pcie, "pcie<X>",
 	       where <X> is the instance number of the pcie from the HW spec.
  - num-lanes as specified in ../designware-pcie.txt
+ - ti,syscon-lane-sel : phandle/offset pair. Phandle to the system control
+			module and the register offset to specify lane
+			selection.
 
 HOST MODE
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