gpio: ixp4xx: Add DT bindings

This adds DT bindings for the IXP4xx GPIO controller.

Cc: devicetree@vger.kernel.org
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>