arm64: dts: add Hi6220's stub clock node

Enable SRAM node and stub clock node for Hi6220, which uses mailbox
channel 1 for CPU's frequency change.

Furthermore, add the CPU clock phandle in CPU's node and using
operating-points-v2 to register operating points. So can be used by
cpufreq-dt driver.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>