cpufreq: mvebu: fix integer to pointer cast

Fix the use of 0 instead of NULL to clk_get() call. This stops the
following warning:

drivers/cpufreq/mvebu-cpufreq.c:73:40: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>