dt-bindings: watchdog: Convert arm,sbsa-gwdt to DT schema

Convert the arm,sbsa-gwdt binding to DT schema format.

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Fu Wei <fu.wei@linaro.org>
Cc: linux-watchdog@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210607194022.3095736-1-robh@kernel.org