cpufreq: pxa: convert to clock API

As the clock settings have been introduced into the clock pxa drivers,
which are now available to change the CPU clock by themselves, remove
the clock handling from this driver, and rely on pxa clock drivers.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>