arm64: dts: foundation-v8: add SBSA Generic Watchdog device node

This can be a example of adding SBSA Generic Watchdog device node
into some dts files for the Soc which contains SBSA Generic Watchdog.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Fu Wei <fu.wei@linaro.org>
[edited subject and moved change to dtsi file]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>