cpufreq: Add Rust-based cpufreq-dt driver

Introduce a Rust-based implementation of the cpufreq-dt driver, covering
most of the functionality provided by the existing C version. Some
features, such as retrieving platform data from `cpufreq-dt-platdev.c`,
are still pending.

The driver has been tested with QEMU, and frequency scaling works as
expected.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>