sh: clk: Use cpufreq_for_each_valid_entry macro for iteration

The cpufreq core now supports the cpufreq_for_each_valid_entry macro
helper for iteration over the cpufreq_frequency_table, so use it.

It should have no functional changes.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>