At early time, the CPU CA73 retention state has been supported on
Hikey960.  Later we found the system has the hang issue and for
resolving this issue Hisilicon released new MCU firmware, but
unfortunately the new MCU firmware has side effect and results in the
CA73 CPU cannot really enter retention state and roll back to WFI state.

After discussion we cannot see the possibility to enable CA73 retention
state anymore on Hikey960, based on this conclusion we should remove
this state supporting from ARM-TF and roll back to WFI state only.  We
will commit one patch to remove CA73 CPU retention state in kernel DT
binding as well.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Kevin Wang <jean.wangtao@linaro.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>