dt-bindings: gpio: Add bindings for Toshiba Visconti GPIO Controller

Add bindings for the Toshiba Visconti GPIO Controller.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>