arm64: dts: hi3660: add pmu dt node for hi3660

Add pmu dt node for hi3660

Signed-off-by: YiPing Xu <xuyiping@hisilicon.com>
Signed-off-by: Zhong Kaihua <zhongkaihua@huawei.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Jumana Mundichipparakkal <jumana.mp@arm.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>