cpufreq: dt-platdev: Support building as module

Make the cpufreq platdev driver as tristate so that it can be built
as loadable module.

Also add MODULE_LICENSE to support building as module.

Signed-off-by: Zhipeng Wang <zhipeng.wang_1@nxp.com>
[ Viresh: Merged two commits, included module.h ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>