cpufreq: highbank-cpufreq: Enable Midway/ECX-2000

Calxeda's new ECX-2000 part uses the same cpufreq interface as highbank,
so add it to the driver's compatibility list.

This is a minor change that can safely be applied to the 3.10 and 3.11
stable trees.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>