watchdog: bcm7038: add device tree binding documentation

Add device tree binding documentation for the watchdog hardware block
on bcm7038 and newer SoCs.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>