powernow-k6: reorder frequencies

This patch reorders reported frequencies from the highest to the lowest,
just like in other frequency drivers.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>