dt/bindings: Document gpio-ingenic

This commit adds documentation for the devicetree bindings of the
gpio-ingenic driver, which handles GPIOs of the Ingenic SoCs
currently supported by the Linux kernel.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>