arm64: dts: hi3660: enable watchdog

This patch is to add watchdog binding for Hi3660 on Hikey960 board.

Cc: Guodong Xu <guodong.xu@linaro.org>
Cc: Zhong Kaihua <zhongkaihua@huawei.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>