dt-bindings: watchdog: Add bindings for RAVE SP watchdog driver

Add Device Tree bindings for RAVE SP watchdog drvier - an MFD cell of
parent RAVE SP driver (documented in
Documentation/devicetree/bindings/mfd/zii,rave-sp.txt).

Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>