cpufreq: armada-37xx: stop using 0 as NULL pointer

Use NULL for NULL pointer to fix the following sparse warning:
drivers/cpufreq/armada-37xx-cpufreq.c:448:32: sparse: warning: Using plain integer as NULL pointer
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/armada-37xx-cpufreq.c

@@ -445,7 +445,7 @@ static int __init armada37xx_cpufreq_driver_init(void)
 		return -ENODEV;
 	}
 
-	clk = clk_get(cpu_dev, 0);
+	clk = clk_get(cpu_dev, NULL);
 	if (IS_ERR(clk)) {
 		dev_err(cpu_dev, "Cannot get clock for CPU0\n");
 		return PTR_ERR(clk);