dt-bindings: watchdog: Add Spreadtrum watchdog documentation

This patch adds the documentation for Spreadtrum watchdog driver.

Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Acked-by: Rob Herring <robh@kernel.org>
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>