dt-bindings: PCI: Add dt-binding to configure PCIe mode

Add "ti,syscon-pcie-mode" dt-binding to hold phandle to the syscon
register that should be used to configure PCIe in RC mode or EP mode.
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NRob Herring <robh@kernel.org>

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

@@ -23,6 +23,8 @@ pcie_msi_intc : Interrupt controller device node for MSI IRQ chip
 
 ti,syscon-pcie-id : phandle to the device control module required to set device
 		    id and vendor id.
+ti,syscon-pcie-mode : phandle to the device control module required to configure
+		      PCI in either RC mode or EP mode.
 
  Example:
 	pcie_msi_intc: msi-interrupt-controller {