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