cpufreq: ti-cpufreq: Use builtin_platform_driver()

This driver can not be built as a module and there is no need of the
platform driver unregister part. Use builtin_platform_driver() instead
of module_platform_driver().

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