Documentation: devicetree: watchdog: da9062/61 watchdog timer binding

Add binding information for DA9062 and DA9061 watchdog.

Example bindings for both DA9062 and DA9061 devices are added. For
the DA9061 device, a fallback compatible line is added as a valid
combination of compatible strings.

The original binding for DA9062 (only) used to reside inside the
Documentation/devicetree/bindings/mfd/da9062.txt MFD document.
The da9062-watchdog section was deleted in that file and replaced
with a link to the new DA9061/62 binding information stored in this
patch.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>