blackfin-cpufreq: Mark cpu_set_cclk() as static

The cpu_set_cclk() function was only used in a single source file so far.
Indicate this setting also by the corresponding linkage specifier.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>