arm64: dts: hi3660: add sp804 timer node

The Hi3660 SoC comes with the sp804 timer in addition to the
architecture timers. These ones are shutdown when reaching a deep idle
states and a backup timer is needed. The sp804 belongs to another power
domain and can fulfill the purpose of replacing temporarily an
architecture timer when the CPU is idle.

Describe it in the device tree, so it can be enabled at boot time.

Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>