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>