cpufreq: cpu0: rename driver and internals to 'cpufreq_dt'

The naming convention of this driver was always under the scanner, people
complained that it should have a more generic name than cpu0, as it manages all
CPUs that are sharing clock lines.

Also, in future it will be modified to support any number of clusters with
separate clock/voltage lines.

Lets rename it to 'cpufreq_dt' from 'cpufreq_cpu0'.

Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>