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

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

Link: https://lore.kernel.org/r/1601444167-11316-2-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>
Acked-by: Rob Herring <robh@kernel.org>

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

@@ -16,6 +16,7 @@ Required properties:
     "dbi"    - controller configuration registers
     "link"   - SoC-specific glue layer registers
     "config" - PCIe configuration space
+    "atu"    - iATU registers for DWC version 4.80 or later
 - clocks: A phandle to the clock gate for PCIe glue layer including
 	the host controller.
 - resets: A phandle to the reset line for PCIe glue layer including