cpufreq-dt: free OPP table created during ->init()

OPP layer now supports freeing of OPPs and we should free them once they aren't
useful anymore.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>