rust: cpumask: Add initial abstractions

Add initial Rust abstractions for struct cpumask, covering a subset of
its APIs. Additional APIs can be added as needed.

These abstractions will be used in upcoming Rust support for cpufreq and
OPP frameworks.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>