dt-bindings: watchdog: drop duplicated GPIO watchdog bindings

Two conversions to DT schema of GPIO watchdog binding happened and came
through different trees.  Merge them into one:
1. Combine maintainers,
2. Use more descriptive property descriptions and constraints from
   gpio-wdt.yaml,
3. Switch to unevaluatedProperties:false, to allow generic watchdog
   properties.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230415095112.51257-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>