dt-bindings: gpio: Add binding for Wii GPIO controller

The Nintendo Wii game console has a GPIO controller, which is used for
the optical disk slot LED, buttons, poweroff, etc. This patch adds a
binding for this GPIO controller.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>