dt-bindings: PCI: Document "atu" reg-names

Document "atu" reg-names required to get the register space for ATU in
Synopsys designware core version >= 4.80.
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>

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

@@ -4,8 +4,11 @@ Required properties:
 - compatible:
 	"snps,dw-pcie" for RC mode;
 	"snps,dw-pcie-ep" for EP mode;
-- reg: Should contain the configuration address space.
-- reg-names: Must be "config" for the PCIe configuration space.
+- reg: For designware cores version < 4.80 contains the configuration
+       address space. For designware core version >= 4.80, contains
+       the configuration and ATU address space
+- reg-names: Must be "config" for the PCIe configuration space and "atu" for
+	     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