Add NULL check  after kcalloc.

Fix below issue reported by coccicheck
./drivers/cpufreq/armada-8k-cpufreq.c:138:1-12: alloc with no test,
possible model on line 151
Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>