arm: imx: Add MODULE_ALIAS for cpufreq

Without this, the imx6q-cpufreq driver isn't loaded
automatically when built as a module

Tested on wandboard quad with a fedora 27 kernel rpm

Signed-off-by: Nicolas Chauvet <kwizart@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>