documentation/devicetree: Move DT bindings from gpio to watchdog

I accidently put the devicetree bindings for the MEN A21 watchdog driver in
Documentation/devicetree/bindings/gpio instead of
Documentation/devicetree/bindings/watchdog, this patch addresses this error.
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@men.de>
Acked-by: NStephen Warren <swarren@nvidia.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Rob Landley <rob@landley.net>