rust: Add bare minimal bindings for clk framework

This adds very basic bindings for the clk framework, implements only
clk_get() and clk_put(). These are the bare minimum bindings required
for many users and are simple enough to add in the first attempt.

These will be used by Rust based cpufreq / OPP core to begin with.

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