cpufreq: dt: Add support for APM X-Gene 2

Add the compatible string for supporting the generic device tree cpufreq-dt
driver on APM's X-Gene 2 SoC.

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