cpufreq: pxa2xx: Add Kconfig entry

Add ability for PXA2xx CPUFreq to be compiled as a module or not at all.

Signed-off-by: Petr Cvek <petr.cvek@tul.cz>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>