selftest: cpufreq: Update MAINTAINERS file

Update MAINTAINERS file with cpufreq's selftest directory.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

MAINTAINERS

@@ -3390,6 +3390,7 @@ B:	https://bugzilla.kernel.org
 F:	Documentation/cpu-freq/
 F:	drivers/cpufreq/
 F:	include/linux/cpufreq.h
+F:	tools/testing/selftests/cpufreq/
 
 CPU FREQUENCY DRIVERS - ARM BIG LITTLE
 M:	Viresh Kumar <viresh.kumar@linaro.org>