cpufreq: imx: Use generic platdev driver

The cpufreq-dt-platdev driver supports creation of cpufreq-dt platform
device now, reuse that and remove similar code from platform code.

Note that the complete routine imx27_dt_init() is removed as

of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);

has same effect as a NULL .init_machine machine callback pointer.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>