OPP code is expanding and is already present in multiple directories
(cpufreq and power). Lets move it to its own directory, to manage it
better.

This also moves/renames the cpufreq_opp file to cpu.c, as it will
contain helpers for cpu device. Its not just about cpufreq, other
frameworks can use OPPs as well.
Reviewed-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>