arm64: dts: hi3660: add power key dts node

We use gpio_034 as power key on hikey960, and set gpio with pull-up
state, when key press the voltage on the gpio will come to lower, and
power key event will be reported.

Signed-off-by: Chen Jun <chenjun14@huawei.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>