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: 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>

drivers/cpufreq/mvebu-cpufreq.c

@@ -70,7 +70,7 @@ static int __init armada_xp_pmsu_cpufreq_init(void)
 			continue;
 		}
 
-		clk = clk_get(cpu_dev, 0);
+		clk = clk_get(cpu_dev, NULL);
 		if (IS_ERR(clk)) {
 			pr_err("Cannot get clock for CPU %d\n", cpu);
 			return PTR_ERR(clk);