cpufreq: enable the DT cpufreq driver on the Integrators

This enables the generic DT and OPP-based cpufreq driver on the
ARM Integrator/AP and Integrator/CP.

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>