dt-bindings: PCI: uniphier-ep: Add iATU register description

In the dt-bindings, "atu" reg-names is required to get the register space
for iATU in Synopsis DWC version 4.80 or later.

Link: https://lore.kernel.org/r/1601444167-11316-3-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/socionext,uniphier-pcie-ep.yaml

@@ -23,14 +23,22 @@ properties:
     const: socionext,uniphier-pro5-pcie-ep
 
   reg:
-    maxItems: 4
+    minItems: 4
+    maxItems: 5
 
   reg-names:
-    items:
-      - const: dbi
-      - const: dbi2
-      - const: link
-      - const: addr_space
+    oneOf:
+      - items:
+        - const: dbi
+        - const: dbi2
+        - const: link
+        - const: addr_space
+      - items:
+        - const: dbi
+        - const: dbi2
+        - const: link
+        - const: addr_space
+        - const: atu
 
   clocks:
     maxItems: 2