cpufreq: armada-8k: Fix parameter type warning

The second parameter of clk_get() is of type 'const char *', so use
NULL instead of the integer 0 to resolve a sparse warning:

drivers/cpufreq/armada-8k-cpufreq.c:60:40: warning: Using plain integer as NULL pointer
drivers/cpufreq/armada-8k-cpufreq.c:168:40: warning: Using plain integer as NULL pointer
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/armada-8k-cpufreq.c

@@ -57,7 +57,7 @@ static void __init armada_8k_get_sharing_cpus(struct clk *cur_clk,
 			continue;
 		}
 
-		clk = clk_get(cpu_dev, 0);
+		clk = clk_get(cpu_dev, NULL);
 		if (IS_ERR(clk)) {
 			pr_warn("Cannot get clock for CPU %d\n", cpu);
 		} else {
@@ -165,7 +165,7 @@ static int __init armada_8k_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);