arm64: dts: hi3660: adb reboot node

Add "hisilicon,hi3660-reboot" node for hi3660.

Eventually when we've transitioned to UEFI this can be dropped.
As we can then use syscon-reboot-mode.

Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Chen Jun <chenjun14@huawei.com>