arm64: dts: hikey960: Add support for syscon-reboot-mode

Add support to hikey960 dts for the syscon-reboot-mode driver.

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