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>