dt: bindings: add documentation for zx2967 family watchdog controller

This patch adds dt-binding documentation for zx2967 family
watchdog controller.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>