dt-bindings: watchdog: gpio: Convert bindings to YAML

Convert the gpio-wdt bindings from text to YAML ones, to permit DT validation.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20221107160338.27910-1-marex@denx.de


[robh: add missing 'always-running']
Signed-off-by: Rob Herring <robh@kernel.org>