cpufreq: enable the DT cpufreq driver on the Ux500

This enables the generic DT and OPP-based cpufreq driver on the
ST-Ericsson Ux500 series.

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