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>
diff --git a/MAINTAINERS b/MAINTAINERS
index 67402bb..56fe62c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2547,7 +2547,7 @@
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-uniphier.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/arm/socionext/uniphier.yaml
-F:	Documentation/devicetree/bindings/gpio/gpio-uniphier.txt
+F:	Documentation/devicetree/bindings/gpio/socionext,uniphier-gpio.yaml
 F:	Documentation/devicetree/bindings/pinctrl/socionext,uniphier-pinctrl.txt
 F:	arch/arm/boot/dts/uniphier*
 F:	arch/arm/include/asm/hardware/cache-uniphier.h