watchdog: digicolor: document device tree binding

Add a device tree binding documentation to the watchdog hardware block on the
Conexant CX92755 SoC. The CX92755 is from the Digicolor SoCs series. Other SoCs
in that series may share the same hardware block.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>