PM / OPP: Move opp core to its own directory

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>