dt-bindings: PCI: Add PCI RC DT binding documentation for AM654

Add devicetree binding documentation for PCIe in RC mode present in
AM654 SoC.
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

@@ -11,7 +11,8 @@ described here as well as properties that are not applicable.
 
 Required Properties:-
 
-compatibility: "ti,keystone-pcie"
+compatibility: Should be "ti,keystone-pcie" for RC on Keystone2 SoC
+	       Should be "ti,am654-pcie-rc" for RC on AM654x SoC
 reg: Three register ranges as listed in the reg-names property
 reg-names: "dbics" for the DesignWare PCIe registers, "app" for the
 	   TI specific application registers, "config" for the
@@ -20,6 +21,9 @@ reg-names: "dbics" for the DesignWare PCIe registers, "app" for the
 pcie_msi_intc : Interrupt controller device node for MSI IRQ chip
 	interrupt-cells: should be set to 1
 	interrupts: GIC interrupt lines connected to PCI MSI interrupt lines
+	(required if the compatible is "ti,keystone-pcie")
+msi-map: As specified in Documentation/devicetree/bindings/pci/pci-msi.txt
+	 (required if the compatible is "ti,am654-pcie-rc".
 
 ti,syscon-pcie-id : phandle to the device control module required to set device
 		    id and vendor id.