cpufreq: dt: Add register helper

Cpufreq-dt currently exports no functions. This means that drivers that
are based on cpufreq-dt have no way of establishing a depmod dependency
on it. This helper allows that link.

Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>