cpufreq: hisilicon: add acpu driver

Add acpu driver for hisilicon SoC, acpu is application processor
subsystem. Currently the acpu has the coupled clock domain for two
clusters, so this driver will directly use cpufreq-dt driver as
backend.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>