dt-bindings: gpio: Convert UniPhier GPIO to json-schema

Convert the UniPhier GPIO controller binding to DT schema format.

I omitted the 'gpio-ranges' property because it is defined in the
dt-schema project (/schemas/gpio/gpio.yaml).

As of writing, the 'gpio-ranges-group-names' is not defined in that
file despite it is a common property described in
Documentation/devicetree/bindings/gpio/gpio.txt
So, I defined it in this schema.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>