cpufreq: imx6q: Select PM_OPP

PM_OPP is a library used by several of the existing cpufreq drivers.
ARM IMX6Q cpufreq driver uses this library for its functionality.
Thus, it should be selected in Kconfig.

Reported-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Nicolas Del Piano <ndel314@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>