dt-bindings: uniphier-system-bus: fix warning in the example

Fix the following warning from 'make dt_binding_check'.

Warning (unit_address_vs_reg): /example-0/system-bus: node has a reg or ranges property, but no unit name
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/bus/socionext,uniphier-system-bus.yaml

@@ -72,7 +72,7 @@ examples:
     // - the UART device is connected at the offset 0x00200000 of CS5 and
     //   mapped to 0x46200000 of the parent bus.
 
-    system-bus {
+    system-bus@58c00000 {
         compatible = "socionext,uniphier-system-bus";
         reg = <0x58c00000 0x400>;
         #address-cells = <2>;