watchdog: add DT bindings for Cortina Gemini

This adds DT bindings for the Cortina systems Gemini SoC
watchdog timer.

Cc: devicetree@vger.kernel.org
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>