dt-bindings: watchdog: Convert GPIO binding to json-schema

Convert the DT binding for GPIO WDT to JSON schema.

Cc: luka.perkov@sartura.hr
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20221125112904.48652-1-robert.marko@sartura.hr


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